First off with respect to MSN:
It really depends what you and your friends use it for. If you just use it for IMing and ONLY that, no other bells and whistles at all then Adium is fine and an extremely slick client. The problem with Adium though is that it's built from (and dependent upon) libgaim which is extremely slow in adding MSN specific features.
MSN for Mac at least has some minor specific things such as personal messages and whatnot (if you need them). If you require things such as webcam/audio chat, sharing folders etc. then there isn't anything for Mac that can do them (yes there is aMSN/Mercury for webcam but they both suck IMO) with the MSN network.
MS Office:
I got the student version of Office 2004 and it works great, and then there's also OpenOffice (requires X11 but in the next few months there'll be a fully native version for Mac which'll be goood) which is good as well

iWork has both Pages (seems to be a kind of DTP/Word Processing hybrid) that at least for word processing seems to be lacking in features compared to Word or OpenOffice. Keynote is amazing and if it wasn't for needing my presentations built in Powerpoint (although it does export to Powerpoint) i'd certainly use it a lot more. TBH I just haven't played around enough in iWork to fully comment on it.
Actual MacBook:
First off I will tell you the hardware problems with my MacBook (Core Duo), that you should be aware of, but ask here and other forums for other people's experiences since most won't be an issue with the newest Core 2 Duo ones.
Random Shutdown - this has been fixed by a software update and so isn't a problem at all now
Palm Rest discolouration - now here's where this was a potentially good thing for me as it got me to see how good they are in Apple Stores. I took it in, and after much attempted cleaning by the genius guy he took it in and replaced the palm rest within an hour

Right now i have some darker colouredness there but i'm thinking it just needs a good clean since it isn't the yellowy colour like before
Flickering Backlight - this one still makes me wonder, as sometime (when only on mains power i think) the backlight will start flicking and then sort itself out again and i won't see it for ages.
Now i've got all the nastyness out of the way there are SO many good things about it. The hardware (apart from the problems above which you most likely won't experience) is fantastic! Magsafe connector, built in microphone and webcam (which as said before doesn't really work with MSN, but works beautifully with Skype), and just the general look and feel of it all makes it certainly seem better made than the Windows laptops i've seen and briefly used. The trackpad with its 2-Fingered scrolling/right clicking is a dream to use and means I occasionally find myself just moving to it randomly instead of my Mighty Mouse.
As has been said earlier and everywhere around this forum Mac OS X is amazing, not that I don't have my minor niggles with it but overall it feels smoother and faster than XP (can't comment on Vista yet...but we have seen Leopard yet either). For 95% of things you can do both on Windows and Mac...then there is another 1% of which I would need Windows for, and 4% which I would actually need Mac OS X for.
Basically hardware-wise you do get amazing quality and thoughtfullness with a MacBook - all you need to decide is what software you want to use and if you'll want to game a lot or not which could sway you to a Windows notebook.