After upgrading to lucid, the system crashes during startup when my Wifi USB MSI US54SE with ZyDAS WLA-54L is attached.
It has been working decently with the previous version 9.10.
My workaround now is to remove the USB Wifi start Ubuntu and than to insert the USB with WLAN.

It works but the performance is very low (especially upload) (max 80kb).
Hence, the second workaround I apply is described here.

Still, the max transfer rate I obtain (SAMBA) is 1MB/s.

I have the impression that problems with USB Wifi do repeat after each Ubuntu upgrade and than are fixed with some updates. I guess, the next few updates will fix the problem. I do hope that this happens soon.