I really like the sound of
Sienna. I think it's pretty, though I'm not sure if I'd actually use it in real life. I can't help but feel it has a slightly downmarket image which would put me off it. I also like
Siena which I feel is more authentic but the
Sienna spelling is much more popular in the UK so I'd probably use that and it looks more complete.
I assume that the popularity of
Sienna Miller is what originally led to its popularity in the UK. It's ranked quite highly here (currently 24) but I've never met one in real life.
A couple of posters mentioned
Sierra. It's never caught on in the UK, possibly because it was a very popular type of car here in the '80s made by
Ford. Very popular as a family car, but not so much as a name.