I always have added mine. Several Arai's, a few Scorpions and a Bell flip face. No problems on any of them. Make sure the helmet is comfortable if you get pre wired helmets. Otherwise just buy a helmet you like. My Scorpion T-1200 has cutouts that fit the speakers perfectly.
I ran Cardo Scala Rider G2"s and G4"s for the past 5 or 6 years. Prior to that I had a wired system on my PC800 using a Star Com system. Wired is nice in that you don't worry about the batteries but then the batteries only become a worry after a long day, normally over 10 hours, of riding and a lot of talking. I don't use the music option so that may shorten the batteries. Cordless makes it much easier getting on and off the bike.
My brother and I have done 10 and 12 hour days with the Cardos still working. We did have trouble last year after many hours in the rain and three of us conversing. There was suddenly a squealing noise that we could not eliminate by shutting off any one of the units. It went away later, probably after something dried out. This year I switched to the Sena 20's as my brother rides with another guy who switched to this model after he and my brother both had repeated problems with the speakers on their Cardo. I never had the problem but I use a full face setup and they use the boom mic.
The little bit I have used the Sena so far has just been to test it and a couple rides with my GPS. It seems easier to program but more stuff to play with. The sound is better. One feature I have not tested is the Sena is supposed to be compatible with other bluetooth units.
The lady friend and I have used intercoms for years and I highly recommend a set of whatever as it is much more enjoyable to be able to discuss dinner, rest stops, that neat house, etc. One thing I I had to train her on is "Bambi" for deer to look at. Saying "Deer" means look out!
My cell phone is linked to my GPS then my headset. If somebody calls me they all say the same thing. They cannot tell I am on the bike.