As far as Hayes ESP boards go, I run on one with virtually no problems.
There is a bug in the board in 16550 emulation mode which causes ints to
be issued whenever characters are received and not at the trigger levels.
The fix for this is to set the "DMA" flag.  I have patches somewhere.

I've also ~80% of a native ESP driver done; work and thesis have pulled
me away from it for a little while.

-john