The basic BT units utilizing ELM327 OBD to UART interpreters are not too bad for needing to monitor a few PID'*, in my experiences when I try to monitor/log data on more than a few basic PID'*, the data seems lagged quite a bit.
I think the best dongles to buy are the ones utilizing the newer STN1110 OBD to UART interpreter, it is a bit faster IC.
Below is a PDF comparison, ELM327 and STN1110 are backwards compatible.
I am not sure if the PLX KIWI uses ELM327, or STN1110, I tried to search for data on which IC they use, but had no luck.
This is the dongle I am going to buy when I can afford to, it definitely utilizes a STN1110 IC.
ScanTool.net LLC - OBDLink Scan Tool/OBD Interface - ScanTool.net
I currently use a chap ELM327 IC BT dongle I bought from E-Bay, and it does work ok for a few PID'*, but I have to use roqyBT4, and roqyOBD4, to get it to communicate with the ScanXL Pro app I run on my I-Phone.
When I am able to get the OBDLink dongle I'll try to compare data transfer speeds, if I can think of a good way to accurately do so.