These devices are quite nice, I've seen lots of people buy them at my weekend job, and we don't have complaints.
Another nice one to look for is GPS-743 which is coming out end of this month. You can check my website for pricing if you need ... but this one has a 4.3" touch screen ... its really nice and bid.
I have a JNC 578. It is great for the purpose of Navigation. It does not have the options of voice downloads, or as a wider map coverage -as standard- when compared to the TomTom One. It competes well with the Garmin Nuvi 200, being R500 or so cheaper. Has local support - which hasn't been great for me as yet, and 2008 maps now available. New maps are very accurate. POIs on this device - 2008 V1.3 software - are pretty good.
My advice if you are looking at a budget navigator - stick with Garmin, TomTom or JNC as they are all well supported. Whatever you do - first ask about loaded coverage maps and upgrade costs. This is the key decision making factor. JNC upgrades are reasonably costly for a budget device.