The new wave of phones will be out in 2-3 months (we aren't exactly sure on release other than Sept-Oct as most likely). So its not a long wait and you will have options with phones using more modern internal hardware with additions like gyroscopes & front facing cameras.
I'd say wait at this point, since Mango's release is so close now, unless you have a little extra spending cash and want to buy a used one now just to get used to & check out the platform until the next wave is released.
The Focus is a very good phone, and is fast as it is let alone how fast it will be with Mango. Its just if you are going to buy one, or more specifically, get one on a contract, I would personally wait a couple months and get one with a more recent generation processor, and more internal storage, that is of coarse, unless you want to toss in a few hundred dollars and mess around with wp7 in the meantime (like I did).
Its all about your budget