They're loud and generally inefficient. There are 3 main types.
1) circulate air within the room. Only good for spot cooling something hot in the room. The hot air will be hotter than the cold air cold so it's no good for cooling the room itself.
2) Hot air exhaust. A portion of the air in the room is cooled and released back into the room. Another portion is used to cool the the condenser and that hot air is vented out of the room. It creates negative pressure within the room and replacement air will come through the cracks in doors & windows. It's inefficient and you may find unit pumping out a decent volume of cool air but the room itself staying surprisingly warm (due to the surrounding external air being pulled back into the room).
3) Closed circuit. I don't know the technical term for it, but it pulls air for cooling the condenser from an external location and then exhausts the air back out as well. It's essentially the same as a normal forced air A/C system except the noisy compressor and fans are all co-located in the room with you.
^both vented types generally have short restrictions on vent types, like 6' to 12' max length (depending on model, with longer lengths being more expensive)
Mits is the gold standard for mini-splits, that's why it is often recommended. They tend to innovate more. They were the first to incorporate variable speed in both the air handler (indoor unit) and the compressor (outdoor unit). They're also the quietest at ~22dB or 24dB. They have a lot of different model types. You can run 3 units off one compressor, or have the indoor unit on the ceiling or in the ceiling hidden from view & reduce noise. For new construction it is quite a bit more expensive than conventional forced air. For retrofit, I can see where it would be worthwhile.
Other options: You might want to look at tapping off an existing line & adding another return over your main heat source. That cost me $700 on my last house. It wasn't perfect, but it was good enough. You should be able to add a 2nd zone to your existing unit for less than a mini-split, but it might not be feasible depending on existing duct work and access to it. A dedicated unit is another option. The hardware does cost more and needs special electric, but It has the same basic lineset requirement as a minisplit and enough capacity for an entire floor (something to consider if you are finishing the basement, for example, with the HT as one of the rooms).
Also, depending how air tight the room is, how many people to have in the room at once, and for how long... you might want to consider bringing in some fresh air.
Edited by rabident - 12/29/12 at 5:30pm