I'm beginning to think I've owned too many amps.
I've owned a Flextone III Plus, an original Flextone XL (2 x 50 watts) and a Duoverb Head which I ran through a Vetta 212 cab. I now own a Valvetronix AD100VT and a Hughes and Kettner Edition Tube.
The Duoverb is probably one of the best amps Line 6 made, and unfortunately, it never took off and was discontinued. It didn't have all the built in effects like the Flextones and Spiders, but sounded MUCH better than the Flextones I owned. If you can grab a used head or combo go for it, but they are incredibly LOUD! The models were better IMHO than the Flextone and it took pedals really well. It was way too loud for me though playing at home.
The Flextone XL (the original) I thought sounded very good. Really warm, and with the 212 cabinet, alot of bass from the bottom end. Good effects, especially the tremelo and chorus. I don't think the models sounded as good as they do on the Valvetronix though, obviously since I opted for the Vox.
The Flextone III Plus isn't a bad amp at all. It can be played loud and also sounds good at "bedroom" levels. Some of the models are very good, while others really lacked some dynamics. My favorite settings were the Twin Reverb, the Cornford, the Bassman, and the 70's and 80's Marshall. The Flextone sounded too much like a digital amp though...you could tell it was solid state. Sometimes I really liked the tone, other times I thought it sounded a bit thin. Maybe a speaker replacement would make a difference. I added a Line 6 extension cab and really it didn't seem to make much of a difference. I definitely like the Vox better.
If you want a Line 6 amp, go for the Vetta II if you want to sink $1600 plus another $269 for the shortboard MIDI controller. A friend of mine has a Vetta II and the sounds are nothing short of amazing. Too many bells and whistles for some though.
The Spiders, I would not recommend, although a Spider III is due out in October I believe. If you want a decent practice amp, then I'd recommend a Spider. For live situations though, definitely not. I just don't think they have the tone of the Flextones or the Vox.
Also, go to www.line6.com to the discussion forums for some good info.