I have a Sony DRU-510UL, the external version. It is very fast, very quiet, and reliable.
I did encountered two minor problems so far. First, I cannot get it to work with Memorex DVD-RW media. I solved that by switching to Philips DVD+RW.
The second problem was that I connect the burner to my desktop using FireWire. I also have a FireWire scanner connected at the same time. If at any time I have the scanner on, my desktop will fail to recognize the burner unless I reboot the PC. It should not be a problem if you use USB2 connection or you have to remember to make sure all other FireWire devices are off before you trun on the PC.
Overall, I like it and will not hesitate to recommend it.