Sure thing, samba.
No need for buyer's remorse w/the 2500. For anyone that purchased it under a $1,000, it's a solid piece of equipment. Right off the top of my head, the 659 as well as the 2500 have preout connections, allowing you to connect an external amp(s) whereas anything lower than the 659 do not.
The 2500's GUI interface is much better for configuration as opposed to the 659s plain text OSD, allows for multiple memory recall settings, the implementation of the YPAO Parametric Room EQ Settings (excellent feature for awkward sized or tough to tame sounding rooms) as well as the amp section being a bit stronger than the 659s (130wpc x 7 as opposed to 100wpc x 7). The 2500 is also THX Select certified if one cares about that sort of thing (I personally do not).
There's a few things the 659 has that the 2500 does not, such as the XM and iPod compatibility features, but I find the 2500 to be much more feature rich and polished than the 659. Again, that's not to say the 659 isn't a great option (like I mentioned earlier, I bought one myself), I just don't find it to be equal to the 2500, then again it really isn't comparing apples to apples. The 659 is a few steps down from the 2500/2600/2700 models, so it shouldn't have the same features and functionality. But for what it does, it does so quite well.
If you really wanted to do a solid comparison, read the 2500's review
here from Clint and the 659's review
here from Gene... -TD