Question: I have various devices to connect to my iMac using good-old RS-232.

Of course, I only have USB connectors on my machine.

I have a couple of Prolific PL2303-based cables, and they seem to work OK, but the kext provided seems a little flaky and I'm not sure about long-term support.

What is the most-stable, best-supported USB-Serial chip-set or cable?

Answer: 
I have two Prolific USB-to-serial adapters but they are made by different Vendors (one is Prolific, the other ATEN).

I've used their supplied drivers just fine. Note that there are open source drivers for these devices too available here: <https://github.com/failberg/osx-pl2303> FYI my use case is actually patching them through to a VirtualBox Windows VM and using them in Windows, so I have two layers of drivers and have not had any problems yet.

However, I think the best supported USB-to-serial devices are those made by Belkin. This is subjective, but I've used them for years without problem, and they are a large company with a reputation. They will likely be around in the future.
I'm professionaly using a
[Roline USB to Serial 0.3 m long adapter](http://shop.roline.com/roline_converter_cable_usb_to_serial_0_3_m/12.02.1089-20.html?t_Action=searchKatalog&t_SearchValue=&t_Sort=&manufacturer=&device=&t_Hier=&t_SubHier=&t_lager=&t_CatID=3078&t_ParentID=3077).
I'm using the [PL2303 OSX driver](http://sourceforge.net/projects/osx-pl2303/).

When I plug my cable, this driver dynamically create `/dev/tty.PL2303-0000nnnn`. I use it from a `Terminal` or `xterm` window with:
`screen /dev/tty.PL2303-0000nnnn`.

0 problem with hundreds of connections on network equipments and servers (Brocade, Cisco, Extreme, Oracle (ex. Sun)…).