I was wary of steam at first but after using it for a few years it doesn't seem to be too bad. Some companies have had horrible launch portals (ICI anyone?) but steam has been pretty reasonable. It has bloated up some in the last few years and has a ton of social gaming and stuff that I have absolutely no use for but which makes it load up quite a bit slower, but it does not appear to be too obnoxious in behavior.
Better yet, when you tell it to quit running, it quits. And if you un-install it, it is pretty much gone gone gone with minimal traces left over and without nuking out other application settings in the process. I don't think it hijacked a single file extension or other application, and it didn't mess with my browser settings or install an uninstallable toolbar or whatever. Pretty well behaved compared to a lot of other portals and apps nowadays.