Yes, the uneven frame/ crop is normal with the Plustek. I have this too - it really is designed, to crop the frame without borders :-(
I use the latest 64bit release of Vuescan with Lion after having installed the Silverfast 8 demo for the needed drivers and it runs fine.
The Plustek scanner can hang from time to time, needing, to switch it off, restart the scanner software and do a pre scan or two, to reset the scanners position - annoying, but thats the way, it is :-(
I would not go through the pain of scanning with the Plustek for effective 3600dpi, as it takes forever and you kick yourself, when it hangs during such a long wait.
I use the Plustek for quick preview scans @ lower resolution and rescan the keepers @ high resolution for print with a proper scanner.
Just, to try it out, I just bought a nice Minolta Elite Scan 5400 with full automatic film transport and autofocus. With the first scans, it's a pleasure, to use.
I will see, how it compares to the Plustek and other scanners in practical use.
One is for sure: it's fantastic, to not have to push the film holder manually through the machine, but lust let it run along all by itself.
People, who complained about the Plustek being loud never heard one of those Minoltas though - those are like diesel generators on your desk (are also built like such - quite the opposite to the Plustek) ;-)
VueScan vs. Silverfast: I really advise everybody, who is stuck with the older, packed SilverFast software, to first try out the latest SilverFast demo, before paying the upgrade fee and if any doubts, straight go ahead and buy a pro license for Ed Hamrick's Vuescan, after having installed the latest Silverfast demo for the drivers to work first.
Vuescan is such a much better, user-friendly and faster to use software, it's a joy!
Make sure, to safe your complete preference profile for each film type/ scanner/ resolution (takes a little time, to prepare)!
Once, you have done this, you simply switch on the scanner, start Vuescan, load a profile and just scan.
I like the fact, that Vuescan also allows, to work with certain RAW camera files (like Nikon NEF files) instead of attaching a scanner!
You can just shoot your negatives on a light table and convert the RAW files into beautiful positive, corrected TIFF files with Vuescan!