Is an offset umbrella the kind that bends and you can adjust it depending on the sun angle? If that's what you're talking about, I'd invest the money on a good one, because I've had some bad experiences with cheaper ones that didn't stay in place properly.
My parents have one in their back yard and they love it.
BFP 1- EDD 2/09/11 Missed MC DX @11 weeks D&C- 7/25/10 BFP 2- EDD 12/22/11 Natural MC @ 5w 2d BFP 3- EDD 1/25/12 DD Josephine born 1/16/12
I like it because you can move it around as you want shade elsewhere - so if you put a kiddie pool up in the backyard, you can shade it, etc. Just be sure to close it and secure it for windy days/etc like you should with any umbrella.