I totally agree with Mike about how these tools free you up to put your heart into it. You can just go for it, even improvise on the fly, and know that you can "fix" it later, either with something like Melodyne or just splicing in another take that was better.
I'm working on a song right now, which is actually pretty difficult to sing. My pitch was all over the place, but I just kept plugging away and did a number of imperfect takes, chose the best one and fixed a few spots with cut and paste from other takes and then followed Mike's advice and used Melodyne to fix individual notes. It's still me singing, but the difference between the original and the "fixed" version is pretty amazing. I'll post a link to it here in the near future. I definitely won't post a link to the original version!