My wife picked one up a week ago, so I can truly compare mine to it side by side and I would say, I like it a lot.
1. Much, much faster.
2. There doesn't appear to be any Lag involved
3. Battery doesn't seem to last very long, but as I tested that part of it early on, it may need a few cycled charges to really say how good/bad it is.
4. GPS is way better than the SGS.
Now, having said all that, the phone is perfect for my wife (she bought the white one). She, having been a long-time blackberry user til her 9700 screen crapped out has been getting used to it, but she's starting to like this phone much better than her blackberry.
Like I said though.. its perfect for my wife. She doesn't know Gingerbread from Shortbread when it comes to this phone, and I intend to keep it that way. Yes this phone runs 2.3.3, but it doesn't seem to bad compared to my SGS with stock 2.3.3 as well. My phone is rooted, but I don't have intentions of rooting hers.
Long story short, its blazing fast, but I wouldn't buy one for myself. Samsung is just too slow on firmware updates to make it worth it for me. In fact any carrier specific phone would be. They are Androids, but they are kind of like Androids bastardized cousin. For me, my next phone is a nexus. I want it to grow as Android grows, and that only happens with Google.