I'm a Symbian user to the heart, but to be fair I tried out both the SMT 5600 and the 8125. I kept the 8125 longer, but in the end I came happily back to Symbian. Here's why:
First, your questions:
1. As stated, there are numerous MSN IM clients available for Symbian. The best, IMO, is AgileMessenger, but they have recently started charging for it, and I don't use MSN that much, so I've resorted to EQO mobile for my AIM needs. It barely gets the job done, but I know there's a Microsoft MSN client available that's a native S60 app, unlike EQO which is Java.
2. QuickOffice (comes pre-installed on all S60 phones) allows you to open and view Word, Excel, and Powerpoint files, just like on WM5. However, you must purchase a full license to be able to edit/create them, again, just like WM5 (IIRC).
3. Personally I prefer the s60 approach because (and this isn't only the browser) it's designed to be used on a phone. I don't like touchscreens cause it's inconvenient for me most of the time to pull the stylus out and try to use that. I can do things much quicker with a joystick and number pad.
Onto my reasons:
1. This is key, to me. I sync everything with my PC (Contacts/Calendar/To-Do/Pics/Videos/SMS/MMS). I do this at least twice a day. With s60, I can do it easily via bluetooth, and I can set it up so that I don't have to do ANYTHING. When I get to my front door, my phone AUTOMATICALLY connects with my PC and syncs all that stuff, so that by the time I get to my computer, it's all there, without me having touched either one. This is key for me.
2. ActiveSync Software sucks. It sucked back in 01 when I had an HP Jornada, and it sucked in 05 when I had the 5600, and it sucked in 06 when I had the 8125. It's just not useful software. It's clunky, it doesn't do things you'd think it should, and it's just crappy.
3. S60, as was mentioned before, is phone first, then PDA. WM5 is basically XP, shrunken. Which means you still have registry issues, you still have an insane amount of folder heirarchy, and you have to know EXACTLY where to go to do stuff like adding ringtones, changing the background, changing settings, etc. With S60, I can put an MP3 in the obviously labeled "Sounds" folder and boom, it shows up in my ringtones list. Themes, I double click on my computer and boom, they install. I can change my menu to look how I want it, and I can change everything else, too.
Things just work out of the box in s60. When I got the 8125, I was so excited, till I realized I had to make all kinds of registry hacks, which if you screw up, you screw up your phone, just to enable/disable things that should've been there anyways.
I also found the software library of s60 to be more extensive, as well as having a more active community.