If you don’t have RW8, you’re not going to be able to open it. It will look like a folder, as soon as you install RW8, it should become a “package.”
I have been using RW8 and can’t say that I’ve seen anything that would be a show stopper that might make me want to go back to RW7. I have both running on the same machine without issue.
The project files(package) are a different format so you can’t switch back and forth with the same project. When you open an RW7 project with RW8, it creates a newly converted project and leaves the old RW7 project as is. If you do need to go back, you would have to reapply any changes made to the RW8 project manually.
I’m not experiencing any “speed” issues with the new RW8 live simulator, in fact, I found it quite fast and a great addition. If you’re using multiple monitors, it’s quite nice having changes made on one monitor reflected on multiple screen sizes almost instantly on the other monitor(s). I know others have reported “performance issues,” but I haven’t seen any.
Most of the issues Still on the 8.1 betas have been related to Apple’s sandbox requirements to get RapidWeaver back in the App Store. There’s still a good number of problems, but I haven’t had anything that made me abandon 8 and go back to 7.