I would definitely recommend going to the States to work this Summer, I went last Summer and was amazed at how different things are compared to the UK.
Unfortunetly you have to have a sponsor like BUNAC, for what they charge they really should do more. E.g. when I was looking for a place to live they just referred me to Craigslist.
I am doing it again this year, going back to where I was last year- Cape Cod, MA. Afterwards I am going to travel the West Coast which I am really excited for, especially as this year, like you, I am in my final year, so I don't have to rush back for the beginning of Uni....I can just stay will my visa expires.
A word of warning, I believe the BUNAC group flight packaging come back at the latest of beginning of October, your visa will last you till early November.
I got a job within three days of being there, so it worked out well for me.
If you work in a service job, the Americans will love your accent = big tips $$$.
So, do it. But be prepared to put in a lot of work yourself and don't expect too much from BUNAC.