One of the days I did was an open pit halfday... Now that was good and bad. The bad was, it was full of racers testing. Officially racers and race bikes aren't allowed but they ignore that completely. So, there were some very fast hombres and occasionally that was offputting. Oh and I suppose I should say, that was the one I crashed on because the sheer amount of laps meant I got really into it and started going too fast
but that's not the fault of the format, just that it's more likely to happen on a longer day's riding.
But the pros are great... You can ride when you want to, stop when you want to. One lap I had visibility issues, came in to change my visor. Do that on a normal 4-session day and that's a quarter of your track time gone maybe, they won't always let you back out. And likewise, someone crashes, red flag goes out, you don't lose track time either. If you're feeling on it, keep going, if you just want to stop or tweak the bike or have a drink or just a break, just do it. Also, even though there were as many bikes out as usual, the absence of synchronised starts and the ability to stop and start meant the track was quiet. It really was a bloody good day.
Not that there's anything wrong with the 4-session, 3-group halfdays but you can see the drawbacks. I think the standard sessions are better for a first time but the open pit days are probably better after that IMO. Though I think more expensive.