I've owned and recorded with 3 different JB3's dating back to late 2002. I recently picked up an iRiver H140 and find it to be a better recording deck than the JB3.
Same experience and findings as stated above: it is smaller, as cheap or cheaper, better battery life, better meters, has digital optical in and out, tested (by me) to be bit accurate, seems at least or more reliable, and no apparent problems with heat related shutdown.
I've heard of 8 hours of record time. I tested my to just shy of 6 1/2, so my experience is that 8 hours would be on the high end, but there has been a bit of variation. And this is with just the stock battery (compared to ~3hr with the single stock battery of the JB3). You can order an ipod battery that fits the iRiver and do a battery swap. I've ordered mine -- less than $20 delivered -- but it hasn't arrived yet. The ipod battery is 2200ma-hr, compared to I think 1300mahr for the stock battery, so it should be good for 10-12 hours of record time.
A huge plus to me is that it seems much better about operating in hot environments. I used mine a couple weeks ago at Wakarusa and had no problems with heat-related shutdowns. I talked to several other tapers using the JB3 that weekend who were experiencing heat-shutdowns, even though their JB3's were out of their bags and under umbrellas for shade. I had my H140 shut up in my closed bag in the direct sun without any problems whatsoever.
I will say that though it is a really nice 16bit recorder, and nice compact 40gb mp3 jukebox (that also plays .wav and .flac files with rockbox!), I don't like the sound of it nearly as much as either my JB3 or my Zen Xtra. Still, for the size and functionality, I'll put up with the reduced playback sound quality. As a bonus though, with it's digital optical out, it is nicer for my headphone playback system, that includes Grado SR225 cans, a Pimeta amp, and a Audio Alchemy DAC. Now I can just plug the H140 into the AA DAC with the optical out and just use the H140 as a bit bucket even for playback.