I was in the same boat as you a little while back, and eventually (sadly) decided to bite the bullet and join the iPod crew with a 160 GB 6th Gen.
Pros:
A great piece of hardware - slender, good looks, decent components
Reasonable price
Readily available
More storage (compared to a 5th gen with Rockbox)
Pretty good functionality with MC (syncing of ratings, number of plays, playlists, etc). Ratings will sync in both directions, even between MP3 on the iPod and FLAC on the desktop, as long as you let MC manage the handheld cache. A really nice feature!
Cons:
Simple and stupid interface
Limited file formats
Occasional hiccups with MC - compilations can easily get fubared, leaving you with thousands of artists and requiring a reflashing of the device (just reinitializing the db doesn't work). It's an intermittant bug in MC12, just upgraded to 13 so don't know if it's really fixed yet
I would have preferred to go with something rockboxable, but in the end it just wasn't worth it.
A quick note on FLAC vs MP3 - trust your ears, not the hype. I've got FLAC on my computer, but I think it's unnecessary for the iPod. Unless you're listening with a really good pair of cans and an external amp, the limitations of the iPod hardware and headphones will greatly outweigh any difference between FLAC and a good quality MP3. Even with really good cans (HD600s) and an external amp, I was hard pressed to find any glaring difference other than general "feel" between a WAV and a 192 VBR. Add to that the fact that you're usually listening to an iPod in what is much less than a critical listening environment, and you end up with FLAC just not being worth it on a portable. For archival purposes or for the high-end home setup, sure, but my end analysis was it didn't make sense on a portable (especially given that I wouldn't be able to fit my entire FLAC library on it!!)
Good luck!