Dave Ive had 3 of these (non VC) but maybe I was just lucky, a lot of other people have also had no issues with the Tamron, think the VC has a couple BF FF issues but builds pretty good.
I loved mine, i also had the 28-75 f/2.8 and that too was super sharp. Its the FF BF stories that put me off getting the VC but i do know i would need something thats stabilised. I will look in to both when the time comes.
35mm is better on crop. The Sigma 35mm 1.4 is superb. Sigma's are known to have awful FF BF issues but when buying new its not so much an issue as you can test or send to Sigma for calibration.
Getting 2 bad copies of the Sigma 30mm f/1.4 is what prompted me to get the 35F2. The first was obviously off, right from the first shot. I had that changed and the second seemed great. I took 100's of great shots with it indoors but when i came to use it as a walk around lens, outdoors, i kept finding i was getting miss focused shots, mainly on medium distance, f/5.6-8 shots. I must have had ti for about 6-9months and then gave up. Its a nicer lens than the 35F2 but l loved how small the 35 was. On a Canon 350D i could fit the camera and lens in to a jacket pocket.
When i got my 30D i brought another Sigma but that was just overall too soft, took it back and got the 50mm f/1.4.
I would really love to try the 30mm f/1.4 again but probably wont buy new. ill track one down that has a proven track record (and save money of course).