Questions about a D3100 as a backup to my D5100/D5000 bodies

I've never considered a D3100, for no good reason except it doesn't have auto exposure bracketing.  Even now the price of it refurbished is so close to a D5100 that, heck, I'd just get another D5100.

D3100 bodies or with the 18-55 VR keep popping up on my local Craigslist, though, at more and more "affordable" prices.

Is the D3100 in fact a very capable camera?  Is it in fact close to my D5100 in high ISO performance?  Any downsides to this little camera?