Had my first experience of Terminal Five recently. The good news first. Checkin for the outbound flight was quick and easy. My bags arrived on the inbound flight and were waiting for me when I arrived at the baggage hall.
The bad news. Security was a nightmare to get through. Lots of staff, lots of lines, but incredibly slow and inefficient. The terminal itself is more than one terminal. We had to use a shuttle train to get to our gate, but that wasn't obvious because they were so busy getting you to shop and eat. The distances you have to walk are enormous. Fine for a fit person, but must be an absolute nightmare for anyone with a fractious child or an elderly person.
Even more bad news. With all of the space, they don't have enough gates, so having walked miles, you then get shunted onto a bus and driven to your airplane. Same on the return journey. They built a new terminal that doesn't have anything like enough space to park enough planes, but has tons of shopping. Oh, and don't even think of trying to connect to a different terminal at Heathrow from there. If you wanted to go from Terminal 5 to Terminal 4 you have to take a train to Terminal 1/2/3 and then wait for another train to Terminal 4. A nightmare. Avoid at all costs, even though they do seem to have sorted the baggage problem.