For me? I learned the harold at UCB. And it may come as a surprise, but I am actually not a super fan of the harold taught at UCB. Well, to clarify, I am not a huge fan of it now. The UCB harold is actually quite dry and stiff, and pretty mechanical. I am specifically talking about doing the Harold that they teach in their classes. Their basic formula is do a pattern game (Boston translation: clover), extract 5 premises, then use 3 of the premises to start the first 3 scenes, and 2 of the premises to start the group games. Each successive beat continues the game in either analogous or time jump manner. That's basically it.
Although I don't personally like doing that kind of a Harold now, I definitely see why they teach it and I agree with it. That kind of Harold is like a longform set on training wheels. With an exact structure and formula that you must follow. It really teaches you how to hone in on a scene, and then how to continue a comedic game idea down the road. This basic concept is so important for any kind of longform structure and beginning improvisors should definitely learn it.
Maybe some of Boston has been rubbing off on me, but as of now, I am definitely digging the more organic discovery kind of loose Harold. I still like game, and second beats, and all that UCB stuff, but fusing it with an organic mentality is really where I am at. Especially with the way Rabbit plays, it's ultra organic, yet we still hit those strong games. It's also why I love monoscenes so much, it's basically a super organic Harold. I do think that without strict training of the UCB harold style, I would find monoscenes to be very unmanageable and difficult. Ironically, it's that strict Harold training that gives me the control to be so loose with the Harold. So in the end, I think the UCB Harold is a great training tool that everyone should do, but for actual shows... I prefer to loosen up a bit!