I've seen them for $319 new but I guess it depends on where you live. If you have to drive too far to get the lower price it isn't worth it.
I have only shot one S&W Sigma. This was in late 2000. I met a guy at the range and he let me shoot his .40 Sigma and I let him shoot my Kahr MK 40. The guy I borrowed it from got it used for around $275. I think for $350, you should be able to get a new one if that's what you want.
I was pleasanlty surprised by his Sigma. Recoil was not bad, fairly accurate,nice trigger pull. Overall, a nice shooter. I have heard alot of bad things about them on this board so I can't tell you too much.
From my limited experience with the borrowed gun, it was pretty decent.