MOXA do good ones. We are a supplier for them but so are many others. http://www.moxa.com/product/UPort_1110.htm /
http://shop.mcidiventi.co.uk/product...639#googlebase


The main issue with some cheap ones is a lack of hardware buffers (important for large data transfers / streams). Some of them don't even implement the handshaking lines.

HTH
Chris