I've got the combo version of the thunder 50 and I love it, its got more than enough gain for what I need it for, I mainly play alterbridge, Chevelle, Tool and Deftones type stuff.
The only problems I have with it is that the clean channel doesn't stay perfectly clean once you go past around 2 o'clock on the gain and it can be a little bright with some pickups, my treble never goes past 1 o'clock.
It also likes to have a bit of mids dialled in (11 o'clock onwards) any less and things get a little less defined, no were near muddy but it just loses 'something'. So for the very scooped tones I use my MXR 10-band eq in the effects loop.
It also responds very well to my wylde overdrive as a boost just to get the extra gain for the very gainy metal stuff, whilst keeping the clean channel clean.
All in all it an awesome amp and the only reason I would change it is if I came into some money to be able afford an Invader or Powerball.