I just installed RockBox yesterday, and so far the biggest improvement is in how much faster it boots. It may seem silly, but I probably get an extra minute or so of listening time each day because of this.
Gapless playback also seem like a big bonus, but I haven't experienced it enough to say for sure (I listen to a lot of epic-length songs).
Didn't the iRiver firmware SUPPOSEDLY have gapless playback implemented? What was up with that? It just didn't work?
I have to admit that at first glance there are a few things in RB that seem confusing (the graphic equalizer for one) but I think overall it seems like an improvement. I think the best thing for people to do is if they have more than one MP3 player that RB will run on to install it on the older or lesser one and give it a try. If my player was brand new I might not be so inclined, but I have an H-120, and if it broke I wouldn't bo SO unhappy to buy a new player
I also have an old Archos HD player as well as an Archos flash playe, and I'm curious to try RB on both of those.