I use it because of an incredibly well-designed interface, and because it's the only way I will ever purchase a game again. I don't have to deal with the retarded DRM that physical copies have, there's an excellent selection of indie games that don't have physical copies, and they have amazing deals on games (especially during the summer.) I understand that some people like owning the manuals/boxes and whatnot, but to each his own. I grew tired of having to lug my CD wallet around just so I could reinstall at a moment's notice, which meant that I wasn't getting any benefit out of the packaging anyway. Since I now have reliable high-speed internet at my house (before last year, my parents still had dial-up >_>) I don't need to worry about bandwidth/DL times for my games. If I want to play a game that isn't installed, I can start the download and come back in an hour or less and immediately start playing.
As for their customer service, I haven't had any particularly large issues that would require me to contact them. The few times I've lost my password I had no trouble resetting it, and my credit card purchases always go smoothly.
As for your question: I'd probably still buy from Steam simply for the convenience and ease of access. I certainly would think more carefully about my purchases, but if there's a game I really wanted and I could afford it, I see no reason to have to deal with Walmart/Gamestop/any other retailer when I can buy it immediately and play it within an hour or less.