4S here
I went to iOS6 and I don't think I'm going back for the following positive reasons since the next jb is still far away and I'm officially unlocked makes leaving jb for a while easier:
1) Siri in Spanish (my preferred method, even though in english it understood me perfectly, my contact names are mostly in spanish so it's easier for it to understand them w/ no issues
2) Since I spend my days at the hospital there are many times someone calls me and I'm w/ a patient and have found that text to answer works good for that.
3) The Extended Siri functions work good
4) Shared photo streams is nice
5) VIP in Mail is good, mostly due to specific sound bites for the VIP I can decide if I need to open an email stat or later
6) Do Not Disturb is a great feature, a little unfinished on what it can do, but is good if you get lots of night calls, not so much if you get lots of emails.
7) Set a song as an Alarm
8) Find My iPhone 'lost' update for iOS 6 is nice if I ever leave it somewhere in the hospital
9) Now that Panorama is part of the camera app I have found is easier to just take a Panorama instead of a normal pic at certain times
10) I travel a good amount and Passbook will come in handy before a jb comes out
11) I don't have ML, but the iCloud tabs would be good too
Why you shouldn't, the most imp points at least for me would be:
1) MAIN reason would be if you use the Map App a lot, Apple's version is really bad. I wouldn't even call it beta, maybe an alpha software
2) You do facetime calls over cellular
3) NC or SBsettings is the one thing I miss the most, but it hasn't bother me too much yet
5) Infinifolders: damn folder limits by Apple are ridiculous, have too many folders now.
6) Emulators: if you love gaming old nes games
7) Remove or Hide the goddamn Newsstand, to this day we still can't put the POS in a folder
8) You use a HotSpot hack
9) Any hack that you use everyday, I already miss Cyntact, ProTube, Home Page in Safari