I always recommend doing it yourself, unless you're a seasoned modder. I have had plenty of soldering experience, and fucked mine up. The contacts are WAY too small! I didn't do chips before, and this is the most advanced thing I've seen. I want to post pics up of the size of these contacts.
Right now, I sent MY xtender to premodded, and they promptly chipped it with one of their used ones. It shipped today, thursday, with their std 2-day $20 shipping. It should arrive Monday, and I'll post my review in the appropriate section. Now they have the enigmahs in, and are shipping their pre-orders.
They included a little warranty with the used xbox, but they complete burn tests for each one. This means running them for 2-3 hours to make sure they don't burn out. My xtender reportedly works great, i'll test with a couple games I "backed up", and that's it.
It's better to buy the chip along with the xbox, that way they can't say it's the chip. Same goes with computer equipment, and anything else. My girlfriend came with her parents, so I can't blame her, etc.