Olpa,
Thanks for the questions!
Integrating the Google Mini was a rather painless process. The documentation that comes along with the appliance as well as the online community provides plenty of in depth information.
Overall, I would say I spent about 3-4 days getting it fully integrated and running smoothly. Most of that time was spent writing the code to parse the XML that the appliance returns.
Show-stoppers? The biggest hangup is a limitation that only lets you get 100 results of the full result set at a time. For our purposes, I needed to have the entire result set available to me, so this was a big issue. We asked Google about this limitation and they mentioned that a fix is on the drawing board (apparently a lot of other search appliance customers have the same complaint).
Overall, working with the Mini was very enjoyable. The results are really good and tune-able, and they are returned super-fast.
Aaron