Recording rehearsals is (IMO) critical for a band to improve. It's extremely difficult to be objective while you're playing. These flash based players make it dead easy. Place it on a flat surface, verify the level setting and press record. At the end, press stop.
When I get home after a rehearsal, I pull out the SD card, put it in my computer, and upload the files to web space.
Sound quality is really good, and the recordings sound very close to what I was getting when I was dragging a bunch of recording gear including very good microphones.
Everyone listens to what they did, and personal pride has each of us fixing what we personally messed up. The improvements in the band have been dramatic in a very short time.
If I was using it solely to record rehearsals, I would have got the H2. The H2's 360 degree micing would have made it easier to pick up everything in a rehearsal room. The H4 has a more standard stereo mic configuration, so with vocal monitors facing the band (and backline), the vocals didn't come through that well. Now, I point a monitor at it - problem solved.
But the H4 lets you plug in external mics, and instruments. It has amp simulation, a tuner and a metronome. So, for an extra $100, I can use it for silent practicing. And if I have a song idea, it takes seconds to fire it up and put it down.
It also can be used as a USB stereo audio interface. It's not great in that application, but it's serviceable. So, I can grab my laptop, toss in a bass and guitar, and record away (including vocals and acoustic guitar with the built in mics).
The other comments on the thread are accurate. Doesn't feel as rugged as some of the competing devices, and the menu system and controls are more complicated than need be.
Quibbles aside, I love mine (a word I don't use often with inanimate objects other than my bass).
js
Zoom H4 Handy Recorder ? 
