We did one about 15 years ago, me & the wife, a friend and the dog hired a, errm, I think it was 65' boat. We had a great time, we set off around 6am most days and chugged through to darkness, covering about twice the expected distance for a 1 week break.
Three is a good number, with one on the tiller, it left two to set locks. When there were no locks around, one was doing food or drinks, one was kicking back... it sounds busy but most of the time it was complete relaxation, working very slowly or just watching the world pass by. Now the kids are of an age, we're talking of doing another.
I'd recommend it, and I'd recommend Viking Afloat, too. Black Prince were very good but, from what I found back then, they were at the pricey but classy end of the market. Definitely worth it if you don't mind paying, as you spend a pot of time on the boats, but we were looking for cheap'n'cheerful. Things might, of course, have changed since the mid-90s.