I did Toronto-London-Helsinki-Singapore (8 hours in Singapore) - Helsinki - London - Toronto. I had about 2-3 hours in London and Helsinki, although on the trip back there was a delay in HEL-LHR, so I ended up being in LHR for ~6 hours. This was all in J.
I thought it was ok. I love movies and I like good lounges. Saw a lot of movies, drank a lot of alcohol, ate a lot of good food, and worked on some personal projects.