I cant seem to find the difference between these two remotes. From a thread a page back, i see:

550:
-Older
-Batteries Just Die
-15 controls

510:
-Newer
-Better Level Meter
-5 Controls

I don't see how they can charge $10 more to loss 10 controls and gain a battery meter. There must be more. Anyone know?

I believe the 550 MSRP was $120. The 510 is a newer entry level model with a bit less features overall at $100 MSRP. So the 510 is actually cheaper, even though you see the 550 deeply discounted many places to make room for the new model. Logitech cut the number of devices (probably cutting the memory cost) to bring the 510 to market at a lower price point, while tweaking the design enough to still make it an attractive upgrade for those who want the new colored row of buttons, battery meter, etc., but may not need more than 5 devices. Logitech probably found that their potential entry level customers are more likely to have 5 devices than 15 anyway. After all, they know exactly what equipment each and every one of their existing customers has, so why add unnecessary cost for features so few will use.

I'd grab one of those cheap 550's while I still could if I were you.

Grabbed one for $57 shipped. I might have been able to get one for $50 at PCRichards if they come back in stock, but I didnt want to miss out and have to spend closer to $70.

Logitech cut the number of devices (probably cutting the memory cost) to bring the 510 to market at a lower price point, while tweaking the design enough to still make it an attractive upgrade for those who want the new colored row of buttons, battery meter, etc., but may not need more than 5 devices. Logitech probably found that their potential entry level customers are more likely to have 5 devices than 15 anyway.
You are most likely correct with regards to the majority of entry level customers not having more than 5 devices (majority probably having TV, cable box, DVD player and/or mini system, AVR). However, when the 510 remote was released, you were able to add as many devices as on a 550 but then shortly thereafter it seems that there was some update released that locked 510 accounts down to 5 devices. I doubt they cut the memory, it seems most likely a software lock in the device firmware or client software.

I just hope i made a good decision getting the old guy rather than the new. Well, if it works, I'm fine.

Just out of curiosity, would could I possibly use all 15 (or even 10) devises for?

I have 11 devices programmed into my Harmony One in the Living Room.

I have 5 devices programmed into the 880 in my bedroom.

What kind of devices are they?

O and the remote shipped in two day. Buying another soon for parents...

Living Room - TV, Receiver, DirecTV, HD DVD, VCR, CD, PS3, Xbox 360, Wii, Fan, Light switch.

Bedroom - TV, DirecTV, DVD, Fan, Light Switch.