I have wanted to build an Extra for a long time. I know that the Goldberg
Extra300 is regarded as the best, but Ebay prices on this kit are pretty steep,
someone is currently asking $500. for one. (***?)
That leaves me with the Great planes kit. I'd like to how it builds and flies.
Is there anything to watch out for? I'd like to power it with a Saito .91.

I have not built the kit but I have flown several trim flights for new guys ... though some years back. I thought it flew quite well overall and considered building one at the time. The only thing that might be a concern is the quality of more recent GP kits. They seem to have gained a reputation for poorly cut and heavy wood. If you could find a kit in a shop near home, you could look it over before purchase. Otherwise, you may have to replace a few pieces of wood that might be considered unusable but that isn't uncommon with most kits these days. On the other hand, you might luck out and get a very nice kit.

It is a great flying plane. I have one that is 13 years old now and is still flying great. It is not built like a new 3d plane but is a great acrobatic plane. I have an old gms .76 in mine the Saito should be fine. I don't think you would regret this plane depending on the type of plane you want to build. I love mine.

A guy in my first club who built one an put a Saito 120 in. He had to put a lot of lead
in the tail to get the CG right. The Saito 100 at 19oz. swinging a 14x8 should be the ticket.