I guess steam is pretty useful. I'm not the biggest fan of eroge steam releases, because they usually get censored there, but we have seen a lot of content patches / 18+ versions being made availalbe as well for the upcoming steam games.
It's a neat place to keep your games in a list, and I like the fact that it keeps tracks of how long you have played etc.
Also, like you mentioned, the steam sales are fantastic. I have spent way too much money on those damn things ...
Just in general for games, it's great, but for VNs it is a bit less of a great place, because of censorship.
All in all I like steam, but if we are talking specifically about VNs, then I perfer my games to be outside of steam, simply because I do not like cut content.
Seeing as I don't play that many "normal" video games anymore, I tend to use steam a lot less. Still, it's neat.