So far I like it. I feel like the battery goes a bit faster than my 6 did. I'm usually near a charger anyhoo so I'm not sure how much this is really going to matter for me.
I really like that Samsung added the ability to add themes right on the phone. There is a Samsung app called Theme Store. It'll give you a background and change the icons to match the theme, pretty cute. I'm not into all those launchers, romming, rooting stuff. So the ability to make my phone all pink easily won a few points.
Other than that so far I am liking it! All of the apps I had on my iPhone I have on the S6. I don't notice any difference between the Android versions and my iPhone versions. Granted I only use really big name apps, Evernote/Bank of America/ tapatalk/facebook/pandora/spotify etc. All look similar to their iPhone counterparts.
I had an S5 for a couple of months as a company phone and enjoyed that but this S6 feels like a nice improvement over the S5 (to me). I don't feel that it's laggy, the fingerprint sensor surprisingly works just about as good as Touch ID on my 6 ( I say "about" because I'm not used to the rectangle button over the circle so this is most likely user error). Also the 64gb version came with about 55gb free out of the box.
Will I stay on it forever? Who knows, I'm taking it a day at a time. I got a 5c as backup in case I decide I just miss iOS too much.
Someone mentioned they didn't know if they should get a 6, if it were me I would wait. I love the 6 but I didn't find it to be a HUGE difference from the 5c. I had to buy it off contract as I was on prepaid and had a hard time justifying that price. I'm now on ATT and put the S6 on Next and will have it paid off shortly just so I'm ready should I want the 6s
EDIT: Also! For you indecisive folks such as myself the S6 takes a nano SIM. This way you can flip flop freely between your iPhone and s6! I did not know this until I was doing the upgrade!