Anyone tried Yosemite on a MacBook? I know the last model in 2010 can run it but I wonder how tough its gonna be. Looked at some threads and some people say its fine while others don't.
As with any such release, your mileage may vary.
It really depends on specific machine configurations and on top of that, user satisfaction can be very variable as performance can be subjective thing. What one user deems as "useable" can wildly differ.
And then you have people who just have other software issues such as third party extensions that may or may not play well with updates. And older machines obviously will accumulate more software cruft.
Then you have people who simple have hardware issues and updates bring them to light.
To be totally honest, the only one true way of really ever knowing is to to try it yourself. Please make backups before updating your software.