I've been in the market for a good Dreamcast SD adapter for a while now, never really liked the idea of buying some hacked cheap plastic chinese dongle. Helder over at the Assembler forums has had some decent internal boards made up.
I just installed one today, and it was a breeze. I was hesitant about taking the DC apart but was pleased to find out it's easier to take apart than an NES. Soldering the adapter board to the serial port was a little tricky, but with the old glob and solder wick technique it was no issue.
Also he shipped it the same day I paid for it, and it came within 3 days.
Overall great piece of hardware!