On contract, it's decent for the price. Snapdragon 80x processor so it'll handle literally anything you throw at it. the one in my HTC is currently playing PSP games emulated to it without a single hiccup. The galaxy line is always solid construction even though it uses a lot of plastic.
Cons are that you can't replace the battery and will most likely drain a little quicker than you iPhone. And no expandable storage via MicroSD. Both of these are nothing new to you since you're coming from an iPhone.
Samsung has reallt decreased the amount of crapware on the phone, and rooting it will allow for far greater customization.
If you want something bigger, I'd hold out for the Galaxy Note Edge which should be out in Sept. Smaller, i'd go with the Galaxy Edge.
Other phones to consider are the HTC One M9 (phenominal camera) and the LG G4. I personally HATE LG, but I've been hearing insanly good things about it.
Also, check this site out. Compares everything on the market.
Samsung Galaxy S6 specs