I'm wavering between a thumbs up and a thumbs sideways, but I'll go with up. She can look really pretty and sexy, but she can also look a bit bedraggled and older than her years. Not surprising perhaps since by all accounts she's a party hard kinda girl.
I haven't seen
G.I. Joe, so I can't comment on her performance in it, but she's proven herself a talented actress in other films. In fact her talent sometimes gets lost and forgotten amidst all the marriage-wrecker tabloid attention she gets. While
Factory Girl had some problems as a film, her lead performance as Edie Sedgwick was outstanding.
Here's the Hugo Boss TV ad with Sienna Miller that was mentioned earlier.