Make sure the software is compatible with your specific unit. Many of the units intended for hiking and other outdoor use won’t do street routing even with the software.
I have a Bluetooth GPS receiver that I got for around $100 (the cheaper ones go for as low as $70) and a Palm Tungsten E2 with crappy mapping software.
The GPS is great – it always knows where I am, it is pretty good at locking quickly, it works fine with the Palm.
I would have sprung for the Tom Tom software but I don’t see that it works with an E2, and I got something (Rand McNally Streetfinder) that was like $22 at Best Buy. It works, but it taxes the Palm, it crashes if you ask it to do much, and it is quite buggy. It could be that I have a low-end Palm, but I suspect other software would be better.
If you are looking for something more versatile than just a handheld GPS receiver (like you want to use it in the car) you may consider one like that. OTOH, my friend and I rented in San Diego this weekend and he got the Tom Tom Navigator, and it was freakin’ awesome for a strange city. Totally worth the $10 a day. Mine, I could tell you exactly where we were, but that one, in a few seconds, could direct you to the nearest Barnes and Noble or breakfast restaurant or anything like that…