I wouldn't myself buy it because I don't want software which anytime I install requires I connect to a server to verify.
You likely won't be installing it very many times, and this is becoming pretty standard for many games now, including non-Steam games.
Theres also the question of security, what if steam program on your pc has holes a hacker can exploit? You have it load on bootup..its active... just like MS OS if theres a hole...it might get exploited and a way in your PC...
So can a LOT of things you have on your computer, your web-browser(s), Live Messenger, banner ads on websites (every once in a blue moon an infected ad slips through the nets, CFC has dealt with them before). Highly unlikely and unlikely to be a risk, heck I've never heard of it happening.
If steam the company shutdowns... your games gone and once it needs "call home" and no answer....no game
A problem with all digital distributions and any game that needs to be activated. However, VALVe (the makers of Steam) have claimed they have a plan in place if they go under. Whether that works out or not who knows, but they are doing very very good buisness with Steam and are not going down anytime soon.
Your game purchase use is at mercy of some other party.
Never had VALVe stop me from playing my games.
I prefer games I get from other companys that use like Digital River for download and selling... you get a serial number must use to install and patch... So I'm at no mercy of any outside party if I need to install my game...legitimate...
Never heard of Digital River but I think I get your point. Paradox Interactive does something like that, you need the serial key to add it to your forum account so you can access certain sub-forums like tech-support. Other than that there is no DRM. They do, however, also sell their games on Steam (as well as GamersGate which they own and in stores).
If steam is whats needed for online playing it should be an option...
installed if need it for that, otherwise for mere installing... should use a solution less intrusive.
Steam isn't very intrusive, a bit forced for those who have not or rarely used it, but fine for others. Both the mods and multiplayer use gamespy or p2p though

However, the in-game Steam Community features are great and some people will really enjoy Steam Cloud.
I still can install my Civ 2, Civ 3 and Civ 4 anytime without need verify it.. to some other party...but this move to be at mercy of a outside party... what occurs 2 years from now if one wants to install Civ 5 on a new PC and Steams closed up or dropped supporting civ 5?
Highly, highly unlikely as I already said. Steam has been around for 6 or 7 years and is only growing in popularity.
As for it a method of preventing pirating... my observation
over 25 years using PC's is every type of CP scheme is defeated
eventually by those who really want to and have the PC savvy...
so it only really messes up us legitimate owners use of our
purchases.
Nobody, even VALVe, believes Steam is the answer to piracy. Civ5 was cracked on Day 1.