So, n+1 arrived yesterday (mumble, mumble, Urban Arrow, mumble). It's fitted with rollerbrakes fore and aft, although came with a disc-ready spare wheel.
After a brief play yesterday, the rollers aren't as horrendous as I was expecting, and the robustness + weatherproofness appeals.
I'm going to find some big 'ole hills to test them on today, but feedback from the collective forum brain also appreciated.
From some fairly extensive Googling, they seem to be universally disliked by people who haven't actually used them, but those who have generally get on with them ok...