Superb deal if you guys want to know really.
I myself was VERY lucky to receive L4D as a Gift from a friend, although I was certainly planning on buying it myself BUT only when its price would go down to around $30, because to be honest even if the game IS great, is DOES lack a LOT of variety in terms of missions/campaigns, weapons and unique bosses. Now, let's consider that the game has been out since a few months and that an upcoming update is going to add variety, new maps and perhaps a few more things that we're not aware of.
With that said I must say that if I hadn't received L4D as a Gift then I would have certainly bought it at $25, that price is EXACTLY what it's worth. It's like... a VERY well done full Half-Life 2 modification in my book (well of course, by using the exact same engine it certainly helps as well, but it's not only that). It IS a great game but I wouldn't have paid $50 for it, ever, but that's just me. Anyway, enough chitchat, $25 for Left 4 Dead is a GREAT deal that everyone should not only consider but take advantage of right now!
Stop reading this and go buy it!