Generally OK, but with three caveats:
1. Connections domestic to other terminals - a complete fiasco. When domestics were in T1 there was no need to go through the whole bag scanning process again. Now, with the exception of connections to T5, you have to go through these unnecessary security steps. This is a crazy backward step that must use a great deal of staff resource!
2. Connections international to domestic - often there is just one immigration officer. I usually seem to end up in a queue behind a Chinese student who has lost their passport. 45 minute wait for a 5 second glance at the passport. Again, crazy. Why not always have an EU/UK nationals queue? Then you end up shoe-horned into a single security queue. No fast track. I now always go out and come back in - far quicker and less stressful, but surely a sign that things are not as they should be?
3. The security. Who invented the scanning machines? I use these as an example with my students of a lack of thought for user interaction. This is multiplied by the attitude of the staff. They appear to believe that the way to detect terrorists is to exert the maximum possible stress on all travellers. If they were guards in Guantano bay, and behaved like that, they'd be in prison now. Disgraceful, frankly.
Good things:
1. the lounges
2. the airside departures area - fantastic architecture in that section)
3. The floor is not sticky (so far), unlike the rest of Heathrow!