Let's break this power conditioner thing down a bit into a sort of Reader's Digest version of things.
First let's look at the surge protection aspect. The vast majority of surge protection devices that you're likely to come in contact with are based on MOV's (Metal Oxide Varistors) and/or SAD's (Silicon Avalanche Diodes) with the former being more prevalent. They work on the principal of shunting the excess voltage to ground. In other words they work like a dike whereby they divert the excess instead of trying to dam it up. The effectiveness of any surge protector, and that includes the BrickWall type devices, is directly related to the proximity to earth ground. A surge is not simply a large voltage swing or spike. A surge, especially due to lightning is a high voltage, high current, and high frequency (ies) phenomenon. As a result, although the overall resistance of your ground wire may be low, to a surge, it looks like a high impedance device. The shorter the distance to earth ground, the lower the impedance.
Surge protectors based on MOV's will typically be rated with a number called joules. The greater the joules, the more hits the MOV can take before it dies. Hence, if that's your only protection, and you're in a quandry between which two devices to pick, go with the higher numbers. Now one might think that a number like 900 joules is a lot. Well if it were located at your breaker, it would be. However for a point-of-use device, that number needs to be broken down. A surge can come in on any of the lines and as a result, one must protect all three scenarios: H-G (hot-ground), H-N (hot-neutral), and N-G (neutral ground). As a result, that 900 joules gets split among the three points of entry. Further, as mentioned above, due to the signficantly high impedance that results from the distance to earth ground, those numbers need to be derated signficantly to arrive at an effective joules. Somewhere around 30-40% less protection is actually available.
Now if you're looking to protect your HT system, then all points of ingress need to run through your point of use device. That includes all plugs and the cable. If you haven't addressed them all, then you leave open the possibility that a surge comes in the back door. **** happens you know.
So what then is power conditioning? It means different things to different people. If you need voltage stabilization, then a voltage stabilizer becomes a power conditioner. If you've got components that have problems with ground leakage, then a balanced power unit becomes a power conditioner. You can see with just these two examples that it's a loose definition.
Most devices always include some sort of emi/rfi filtering. This is achieved by things like inductors, capacitors, and other types of devices. They're intended to remove high frequency junk that might be on your AC line. Regretfully, there is no standard when reporting this quantitatively. Some companies specify it as dB (50 db FWIW is about 99.7% reduction) or as a percentage. Some further tell you over what frequency range they're talking about. Some only tell you at one particular frequency. Monster, as far as I know, doesn't say much about it other than they have it. Panamax is bit better. You'll find devices that are also sold for computer applications to be quite a bit more forthcoming and generally one or two emails gets you some fairly detailed information if that's what you're looking for.
Now this emi/rfi filtering can be achieved in a number of ways. It can come right after the power cord or it can exist between pairs of outlets. The reason for the latter configuration is to address the concerns of people who are worried about Electronic Component #1 dumping some rfi back down the power cord. In the latter scenario, the further away you are from any one component, the greater the emi/rfi filtering is. It's cumulative if you will. Hence if one's got a device with that incorporates some high frequency switching like an SACD player, then you'd put that on the first pair of outlets after the power cord and stick your TV on the furthest one. Make sense?
Price is no indicator of effectiveness here. Numbers are. One can spend hundreds for a 1250 joule unit and $30 for a 3000 joule one. Which one do you think has a better chance of protecting your equipment over the long run? On the other hand, which accountant do you think is smiling more?
Protected Equipment Warranties are useless in my opinion. If you don't have renter's insurance you ought to get it and make sure it includes provisions for Full Equipment Replacement in the event of surge damage (theft, accident, water, etc. too) instead of this prorated stuff. With a good policy, if a surge trashes your stuff, you'll be back here laughing and talking about how you need to upgrade and be asking for suggestions.
The more expensive Monster and Panamax units provide features which you may or may not find useful. Some have been touched upon by other posts in this thread. Appearance, large # of outlets, triggers, switched and unswitched outlets, delay turn ons, etc. may or may not have importance to you. If they are, then be prepared to pay accordingly for your needs...errrr...wants...errrr...you know
Now comes the question. Why do some people observe an improvement in their display when using such a device and other don't? First of all, I don't think it has to do with large voltage swings. If that was happening, then you'd notice it with your lights and unless those are dimming and/or running bright, then you can say with reasonable assurance, that your power is fairly constant. So we scratch that.
What about RFI/EMI on the lines? That's "possible" but you'd have to have extraordinarily large amounts that simply overwhelmed your power supply. Fairly unlikely but seeing as how virtually any device you buy is going to have such protection then one can reasonably expect that to be dealt with fairly effectively by anything you buy.
The most likely reason, in my opinion, is that by running everything through one device, you largely address the problems with ground loops. Sometimes these ground loops are severe enough that even running them through one device still doesn't deal with things like hum coming out of your subwoofer or speakers. In those cases, Calrad sells a video isolation transformer that renders that a non issue. As a side benefit it also cleans up the video.
I don't have any specific recommendations for products. If you need the additional features then look at the Panamax or Monster units. I happen to lean towards Panamax but that's mostly because I think they're more forthcoming with specs. Monster just confuses everything and trying to get information by email or phone is like trying to score with a girl while her father sits with a shotgun looking at you. It just ain't going to happen.
If you don't need those capabilities, then a host of products are out there. For around $20 the Stratitec looks good. Doesn't have the in-between-outlet emi/rfi but the rest of the specs look good. Trip Lites Isobar series is very nice. Priced in the vicinity of $60, you get in-between-outlet emi/rfi, decent joules, and a very nice feature called sine wave tracking. This limits the let through voltage by constantly tracking the AC waveform. For a unit based on SAD's, the DPS series from Transtector is nice. Quicker response time than MOV's and quite excellent at catching little baby transients...call them spikes. That's around $100 straight from the manufacturer which happens to be a sister company to Polyphaser, probably the industry yardstick when it comes to serious protection from lightning.
Things you can do if you're renting would be to check your outlets with an outlet tester (around $4 at Home Depot) to make sure you're wired correctly. Also, if you have access to the breakers, kill them in the room where your HT is, remove the outlet, pull the wires out, clean them, and wrap them around the screws instead of stabbing them through those little holes. It's a better, more robust connection. Then also add the Calrad unit which is around $10. That, to me, will give you the greatest bang for the buck and save you some cash at the same time.