I've got a BT-338 and it's great. The first week I had it, I flew to NYC and had some problems getting it to pick up in Manhattan, but I had no clue what I was doing really and the "jet lag" GPS units can experience when being turned on one place, then being turned on 800 miles away the next time.
Whatever you get, be sure to get a SiRF Star III chipset. I heard about some newer ones on the near horizon coming out, but right now SiRF is hands-down the way to go. A lot of SiRF Star III receivers are basically the same internally and operate near identically (but the BT-338 was cheap enough and had gotten such great reviews that I figured, why get something else?).
For the record, I got mine at BuyGPSNow.com and used the coupon code BGN01, which seems to have been valid for a long time. Got mine last December, still holding up great. Customer service was great, too ... when I bought mine, I got two - one for me and one for the bro. Both came with free vent mounts. They had a deal where you could get some nicer power mounts with like $10 or $20 off if you bought them at the same time as your GPS receiver.
Well, I forgot to add it when buying the second unit. Upon receiving it, I wrote BuyGPSNow.com and asked if I could still get the discount, and they said sure - just order it and give them the order number, and they'll credit the difference to the bill afterwards. Worked like a charm. =D
I spent a while reading up on PDA and GPS navigation over at
www.gpspassion.com before I bought anything. Give it a look-see. Kinda confusing to navigate when I was there, but the forums had good info and helpful people, and the articles were good (when I could find the up to date ones).
For the record, I'm using TomTom5. It gets a little confused sometimes with all the construction we've had (and once told me to make three right turns at an intersection instead of just a left turn ... crazy little thing), but all in all I've loved it. Pretty maps, good voice direction, very accurate 99% of the time, and when you make a different turn than it has instructed, it's pretty fast in recalculating your route.
I used it on my way to a wedding just outside Knoxville, actually. I told it I wanted the fastest route possible, and it had me turn off the main road and start taking all these crazy backroads. I thought for sure I was screwed until I finally hit the next big road ... about 200 yards away from the church, and about 300 yards in front of the people I'd been behind before I turned off originally.