2) Saves battery life of the phone if you use both (I have phone for Spotify and watch for stats). GPS eats phone battery.
3) If you're training for a race you can upload your whole training plan to the Garmin and it knows what you are doing on each day and takes you step by step through each activity. It's brilliant for interval sessions.
4) Can combine it with a footpod to give you cadence stats.
EDIT: Oh should mention that once the data is on Gamin Connect you can do what you like with it. I started out with Nike+ and my friends use Nike+ on their phones so I stick my Garmin runs into a converter and it uploads it to the Nike+ website. I'm guessing you can do the same to Endomondo if you still want to track your stats there.
I've been using the Nike+ website and running app for the last year and really enjoyed it. I have some friends that use it as well. The achievement debacle looks like it might be over now? I got the Cinco De Mayo one today and the birthday and anniversary ones about a month ago.
I've stopped using the app though as I got myself a Garmin 610. There is a website that converts Garmin activities to Nike+ and it's as simple as cut and pasting the Garmin activity URL into a field. So now I have the additional data from the Garmin system but can still keep my Nike+ profile updated. Don't feel that you have to get a Nike watch to keep using the Nike+ website.
I've tried four pairs in the last year but have settled with an excellent pair from Shure. Here are my impressions of all four pairs, hopefully this is helpful info.
My Sennheiser IE-60's were excellent sound quality but not suitable for running at all. They would just not stay in. I use these when I'm not running.
I tried the Sennheiser/Adidas sporting ones, I think the model prior to the CX680's and they cost around £30-£40. They fit well but the sound quality was nothing to write home about.
I moved on to the Denon AH-W150 Exercise Freak bluetooth headset at £150. Having no wires was very convenient and I had no issues with sound cutting out. The fit wasn't great though. They are in-ear headphones and sound ok but nowhere near good enough for that price. The seal loosens after a running a few paces and the sound quality deteriorates drastically. I wouldn't recommend these at all.
I'm currently using wired phones which I'm very happy with. They are Shure SE215's, in-ear headphones at £75. Once in, these things aren't going anywhere. They are a great fit and the seal is perfect. The sound quality is very good as well, possibly a little too muddy in the mid range with excessive bass but much better than you would expect at that price. The only downside is that you can hear very little of the outside world and have to be super carefull at road crossings.