Tom got a lot of flak over the years. But a lot of it came from people who tuned in to S1, maybe S2, and then never came back.
As an actor, he came into the show with very little acting experience and he matured and grew a huge amount in the 10 years between the Season 1 Premiere, and the Finale. There's a noticeable different in his acting ability in the early seasons and the later ones. Physically, he also matured a huge amount and whilst he had very much a young teenage look in Season 1, by Season 10 he was definitely looking more like Superman than Superboy.
I've got a lot of respect for actors who don't just pick up a paycheque and then head home, or to a party to get high, or whatever they happen to do outside their job. Tom put in a huge amount of hours on the show from day one, and many other actors on the show have mentioned in interviews how hard he worked - above and beyond what he was required to do. And then to top it off, he took on an executive producer role after sitting in production meetings (which he wasn't really meant to be at) at the back of the room just to get some experience. The guy really did just live, work and breathe Smallville for 10 years.
I also think that he's carried the Superman mantle with a lot of dignity in his personal life, just as many of the other Superman actors have done before him. No scandals, happily married, pleasant and nice guy from all accounts. Gives a lot to charities in his spare time but keeps it very low key - there was a story one time about how him and his wife spend a few thousand on Xmas presents every year for a children's charity, but they always do it very discreetly and avoid publicity over it.
So, thumbs up for Mr Welling - if I ever bumped into him in a bar, he's one of the few celebs I'd be honoured to buy a beer. I hope he's successful in whatever future projects he's involved in.