+1 on the Nuvi 550...be aware that there's no Bluetooth.
When looking at less expensive alternatives to the Zumos, be sure and note if the units will allow custom routes...not all of the cheaper ones do (assuming that feature is important to you).
Another approach you might want to consider is using one of the smart phone-based apps. I use GPS Drive, have used the TomTom app with good success, and of course Apple is (in)famous for their new app. I've heard great things about Navigon, although I have not used it myself.
None of these allow custom routes that I am aware of.
If you think about going the phone route, be sure to consider:
- power - GPS apps will drain your battery really fast
- heat - running the GPS apps constantly will make the phone heat up, which can be an issue if you're trying to put in something waterproof.
I made several long trips with my iPhone mounted on a RAM mount as my GPS device, and it worked reasonably well. I had to keep the display turned off during long stretches with no turns to keep the power drain manageable (even when plugged in, it would gradually discharge the battery with the display on all the time). And I would just stash it in the trunk if it started raining.