I think display options are the biggest restriction with low-end Mac minis, at least for my use. These low-end Mac minis have only 32MB of graphics memory and that's not quite enough if you want FullHD resolutions and/or enhanced display modes. But if you're OK with something like 1280x1024, then they may be OK, but with caution...

I'm still using the only Mac mini model with 64MB graphics memory (the silently upgraded 1.5GHz one) as my daily driver, and it's fine on a FullHD display with DVI connection. I've had couple 1.42GHz minis, but they both had issues with DVI/FullHD.. first, 32MB isn't quite comfortable for that and secondly, they had some blackouts etc. which were annoying to deal with. You could get them better by tuning frequencies by hand or using (blurry) VGA, but still it felt that there is some kind of hardware issue. Two 1.5GHz models I've had have been just fine on everything I've thrown them, I think there's some kind of improvement on their display side...

Otherwise Mac minis could be usable for all kinds of things, but I myself would stay away from anything less than 64MB graphics mem...