I'm a Brit and love the original series.
That said, I prefer the US version because there is a lot more to it.
The original show focuses on David, Gareth, Tim and Dawn (Michael, Dwight, Jim and Pam) and only has 12 episodes. 14 if you count the Christmas specials which came after.
There are 201 episodes of the US Office. The peripheral characters become really well developed and there is a lot more breadth and depth to it.
It has a different feel to the UK version. It's more overtly a sitcom rather than trying to present itself as a legit documentary.
Like a lot of shows, it started to get shit towards the end as they ran out of ideas and tried to compensate by doing increasingly stupid stuff. It's a sad end that many US shows suffer. On the contrary, the UK version ended well. The Christmas specials basically kill off the 2 main story arcs.
All in all, I think the US Office is a great show. Some of the writing and performances are top drawer. I think the hallmark of a great comedy is when you genuinely care about the characters, and I really was invested in several of them.