Ubuntu 10.10 was nearly flawless.
Upgraded to 11.04 and lost the ability to change screen brightness easily.
There are some configuration recommendations on the Ubuntu wiki specific to the MacBook that you should take note of.
The nvidia display configuration tool is finicky; unplugging from an external monitor at my desk and plugging into a projector in a conference room sometimes takes multiple rounds of invoking it and clicking "detect displays." Also there are a couple of projectors it won't play with.
I used grub-pc (the default) not grub-efi. If you don't have to dual boot Windows you don't need Boot Camp, ReFit nor the ridiculous hack of MBR partition table syncing. Just carve out some space with Mac OSX's partition tool and have Ubuntu install its bootloader to sda. Hold down option when you boot to choose your OS.
Don't blow away the OS X partition, you apparently still need it for firmware updates.
If I had the option, I'd get a PC instead. I like Asus, and I'm aware of more than one fan of Lenovo but never used them myself.