I've talked about my experiences with the ihp-120 previously. It's a great piece of hardware, but iRiver really fumbled the software, and though they promised updates to rectify some of the more glaring omissions, nothing was ever released, and the product line has since been eclipsed by newer models. This left many users (myself included) feeling burned.
I ended up returning mine for a Rio Karma, which is a wonderful player, but Rio's closed up shop, and their promised next-generation players have gone from vaporware to ghostware.
A few months ago, a friend of mine decided to ditch his ihp-120 for one of the new Iaudio players, so I bought it off him for $40. At the very least, it comes in handy as a 20GB portable drive. I never expected to use it as a music player.
Enter Rockbox, the folks who wrote a wonderful open-source firmware for the old Archos series. They've written new firmware for the ihp series that not only rectifies iRiver's shortcomings, but adds several new features as well.
It's still a work in progress, but so far, the Rockbox firmware allows gapless playback on Ogg files, something only the Karma had done thus far. Ogg files don't have the same partial-frame cutoffs that mp3s do, so it should play seamlessly, but one of the problems with the original ihp firmware was the shoddy job it did with hard-disk caching. This has been fixed in the Rockbox firmware, and I've tested it on several files, finding that it does in fact work. Nice.
Several other features include on-the-fly playlisting, Flac playback (also gapless), and a customizable user interface. This is still developmental stuff, so the recording features are a bit rough at the moment, and the FM tuner is still rudimentary, but the Rockbox firmware already allows direct recording from FM radio–something iRiver's engineers claimed was impossible on the unit.
Rockbox is working towards milking all it can out of the hardware on the ihp, and even in alpha, the results are quite impressive.