Going well in my recent experience. I came in to T5 on BA138 from BOM last week and was through immigration and reunited with my luggage within 30 minutes of landing.

Signposting could still be improved airside and the wait for lifts in some areas (e.g. from arrivals to the BA arrivals lounge) was too long in my experience.

But overall, T5 is now one of the best large airport terminals in the world.