I've run two (and am considering my third).
Manchester last April:
Tough training in the cold and dark, but it toughens you up.
Nice wide roads, really flat elevation profile.
Most of the route is through populated areas and it's very well-supported. The atmosphere is great.
York last week:
Tough training in the heat (never happy). Tougher than it was during the winter, to be honest.
Thin country roads with uneven camber. Lots of long gradients uphill.
The city part is well-supported but the rest of the course is quiet.
Perhaps Edinburgh next May:
Seems like a nice time of year to hit reasonable training weather.
The course is an overall descent, and it's supposed to be good for PBs and fast times.
Apparently, very festive atmosphere.