I did my twin rating many moons ago in Haverfordwest at the same time as I was doing my IMC. The twin rating was in a lovely Twin Commanche (G-ASSR, now since deceased unfortunately) and the IMC was in a Cessna 150. The difference in speeds and handling were interesting but, because the aircraft were so different, didn't cause any problems if that makes any sense.
It was a great experience being "junior airline pilot" with a handfull of levers and I am really glad I did it. As has been mentioned, maintaining the rating (or whatever it's called these days) is hideously expensive on the following scale:
1. Simple singles = throwing £5 notes on the fire
2. Complex Singles = £10 notes on the fire
3. Twins = £20 notes getting singed
4. Helicopters = £50 notes burning.
I can just afford to be throwing the fivers on the fire so I didn't continue much beyond the rating.
Do it, you will thoroughly enjoy it and it will improve your piloting skills as will doing the IMC.
PS This was my first experience with put-away-wheels and VP props