Windows Live Messenger is much improved over the old MSN but I'm sure there will be people either sad to see it go or who will have a lot of memories.
In many ways MSN changed the way we communicate or allowed us to accept various forms of instant communication. MSN , SMS, Twitter are all part of the same package.
Still I know heaps of people who have never used a MSN type of tool and surprisingly it is a tool which never successfully infiltrated business. I always thought that it (talking generically) was an ideal form of communication in small business especially.
I moved over to Digsby
a year or so ago and although I check out WinLiveMess now-and-again Digsby is a best in class IMO.
Digsby has just updated too - and its memory use is small, its task bar notification of all your emails, twitter and Facebook is fantastic. It looks great, runs flawlessly in 7 and obviously, connects with all your other IM accounts. Pidgin
is open source and if you just want IM, is highly regarded. I presume its fine in 7.