Food for thought on Shake
If you don't have any other use for a compositing program except for a CK, Shake is going to be complete overkill.
The last version of Shake for mac was meant to run on Leopard. It may, or may not run well on your system. Snow Leopard is doable, and some have even reported getting it to run on Lion. The farther you get from Leopard, the better the chance it won't run, or that an OS update might break it. To have the best odds of a successful Shake installation, you should do a clean install and Shake should be loaded before FCS. If you are at a point of freezing your system at a particular level, it might be something to consider. But if you need to keep your OS up to date to support other apps, it might not be that great for you.
Also the UI, and workflow is quite different. Similar to Color with a node based UI. Even just doing a simple CK might prove to be a challenge for some new users. I'm not saying it can't be done. Just that it's a lot more work than installing a FCP plugin.
On the plus side, it can be had for bargain prices.
Another option is to farm out the shots that are giving you trouble to someone that has Shake or other compositing software.
Scott Sheriff
Director
http://www.sstdigitalmedia.com
"If you think it's expensive to hire a professional to do the job, wait until you hire an amateur." ---Red Adair
Where were you on 6/21?