I know that there are beginning to be several mounting plates available for cameras like the F3 to add things like external recorders. Zacuto has the Zwiss Plate. I need to get one and was wondering about what all the options were available. The Zacuto is $140.
First you need to decide what you are trying to do. If its adding accessories up top the Berkly (? spelling) plate is nice. Then see what works and what you can afford.
In terms of a full system of rails and bits, these two are the most complete. There is Jag35, VF Gadgets, Element Technica, RED (as in the camera) and a few others that have very specialized bits for Steadicam Ops. Oh, a couple I forgot about... specifically for the F3 there is the Anton Bauer battery mounting plate that can operate on 15mm and studio spaced rails and there is also one from Redrock Mircro.
Duke's question is bang on. You need to spend A LOT of time on both Berkey and Zacuto, mapping out what you are trying to achieve with what gear and develop scenarios. There is no easy way around this other than putting in the time mapping things out on your own.
If there is something you cannot find then find a local machinist, talk through your needs with some sketches and get it made.
The Zacuto one you mention is one of their more economical solutions. The fact you can attach rails to the top of it is a bonus. I can see a lot of potential use for this. Even getting 2 of them to create a cage.
The simple question is what are you trying to accomplish? Knowing that a decent recommendation can be made. Do you need a mattebox, follow focus, on board monitor, on-board recorder, accessory battery mount, wireless video transmitter, wireless audio receiver, video converter box, handgrips, etc., etc. Without this knowledge any answer is just a guess.