I was going to write this post last year when Mathematica 6 came out, but I've managed to procrastinate all the way to the release of Mathematica 7.
I have always found Mathematica intriguing; I still remember magazine articles about the first release back in the 80s. Naturally its mathematical capabilities are notable, but it's as a programming environment that I find Mathematica most exciting. When I watch some of the screencasts, I think “yes, that is what programming a computer should be like”. This one is a good example. I rarely feel a similar reaction to more mainstream computing research, and perhaps only the work from Xerox Parc's golden age matches it.
But despite that, I don't have much inclination to actually use Mathematica. I've played with it on at least one occasion, and wouldn't mind doing so again if an opportunity arises. But I have deep reservations about the proprietary and closed nature of Mathematica as a product.