Let me say what most posters who come here with issues, looking for some help, don't want to hear.
FCP X is working for me.
Now let me try and offer some suggestions that might help.
Save Isn't Working:
There is a way to check if it stops working. If you can no longer Undo something, Save is not working. This seemed to be a big issue in 10.0.0, but it seems to be gone in 10.0.1. I have yet to see it. If you discover that it does quit working, you might be able to duplicate your project. No need to duplicate render files, just the project. Easy to do. I don't know if it works, since I havne't had this issue.
RAM:
On my system, 8GB would not be enough. I'm running Lion 10.7.2 and FCP X 10.0.1. FCP X typically takes up over 7 GB, with a kernel taks taking over 1 GB. That's with nothing else running. Running out of RAM for FCP X does seem to be an issue for some people. Should it be an issue? No. But having more than 16 GB is a very good idea.
Inactive Memory:
I don't know what causes Inactive Memory to grow, but it does. While I use FCP X I watch Activity Monitor. I open Activity Monitor, Hide it and right click it to Show Memory Usage. That way, I can easily check the status via the Dock. Inactive Memory grows to over 4 or 5 GB. From many other posts I've read, this appears to be an issue with Lion. Some users have gotten memory cleaner applications for this. I use purge, a command in Terminal. If I don't reset Inactive Memory, I will encounter issues. (at least for me)
Preferences:
Whenever you start to have issues, most agree that trashing your FCP X perferences helps. Get Preference Manger. It's free and it not only can easily trash your preference, but it can re-load them as well. Obviously re-load known good ones.
http://www.digitalrebellion.com/prefman/
Also, here's a nice troubleshooting link for FCP X.
http://www.kenstone.net/fcp_homepage/troubleshooting_fcp_x_taylor.html
As I mentioned at the start of my post, FCP X is working for me. It's not perfect. I've had it freeze. But, I've never lost more than a very small amount of changes. What I'm currently working on is broken down into 10 different projects. The single Event has 1,914 clips of 720P HD. I spend hours editing every day. I periodically export both ProRes and burn BluRay for viewing, testing and backup.
While FCP X is not perfect, it does work a lot better than what you are experiencing. I hope you'll find some of these suggestions useful.