Back when I tried that I simply used xbench - the 4k block writes went below 10MB/s. I don't have the exact numbers, but I do remember that mac os took > 30 seconds of spinning wheel to boot, compared to 1-2 usually. BTW, filling the disk is as easy as performing "erase free space" with Disk Utility.
That doesn't seem typical - I'm using an 160GB X25-M G2 and have almost certainly filled the drive at this point (been running with less than 15% drive space free for a while, I leave hibernation on, installed and removed Boot Camp, etc.) and I'm still getting ~55 - 60MB/sec 4k sequential writes and ~60MB/sec 4KB random writes with the latest firmware in Xbench. I'd think booting would be mostly reads with a few writes here and there, too...
Although the 160GB has a slight write advantage over my 80GB G2, it appears those last 15% make quite a difference. That wouldn't be totally out of line from how regular HDs behave once they approach 100% utilized capacity.
SSDs (the intel ones at least) have a buffer of cells that are inaccessible to the OS (that buffer is larger on e models), maybe that's double for the 160?
