I have one and its one of my fave piercings. It can take a long time to heal though. While healing certain sitting positions can be uncomfortable especially if you ride a bicycle or a motorcycle. On long bicycle rides I sometimes still take mine out.
My only regret is not getting it pierced deeper so I could stretch the size up more. As it is 8 gauge is the biggest Ive had in it but usually keep a 10 gauge ring in it.
I had six 0g rings in my balls. I always wanted a guiche but, like bare_lee said, while healing certain sitting positions can be uncomfortable. Also just sitting on a hard chair or wooden stool, you are constantly adjusting your self trying to get comfortable. I often wish I would have done it but other times I'm happy that I didn't.
I have one and love it. I had some healing issues for a while = which were a bitch. Living in a hot country, hot and sweaty down there, slow to heal, etc. Try and get pierced as big as possible, and go for a curved barbell - not a ring. That'll mean less movement, assisting healing. Use saline in a shot glass to twice daily bath it.
Mine was pierced around a 12 g in 2011- I'm now sporting a 4g! Biggest it has ever been. I cycled for 5 hours yesterday.
Once it heals its an amazing piercing. While sitting you can just adjust your position just a little and what once hurt while healing is now something else entirely. Its a nice feeling to say the least. Cycling is only a problem for longer road cycle trips (more that 40 miles for me) and I take it out. I done a few 100 milers and I take the guiche out (because of the narrow seat) and sometimes navel (if wearing shorts or bibs that cover it).
On my motorcycle the guiche is very nice when the seat it vibrating beneath me especially at higher speeds. Thinking about this, I think I probably get more enjoyment from the guiche than any other piercing.