I really encourage anyone who is watching this for the first time NOT to read the spoiler above my post as it spoils the entire series. This is definitely one show where the mystery is more important than the solution. Plus, I've heard rumors that they're going to handle it differently and have already been signed on for more episodes than the British version.
I watched the British version as well and I was very, very pleasantly surprised with the American version. I think what was missing for me as an American was the powerful punch of nostalgia, and an Americanized version does that well for me. Watching the British version I could tell there were tons of cultural throwaway lines but it doesn't have the same impact as feeling them.
One thing they did well was keep the sort of fun spirit that the British version had.
I loved John Simm as Sam Tyler and his are big shoes to fill, but I was also pleasantly surprised to not find it hard to switch over to Jason O'Mara. The one character who hasn't quite settled in for me is Harvey Keitel. The UK actor who played that part was just brilliant (even though I don't know his name) and just lived and breathed that setting. HK doesn't seem as natural in that part to me.