I have the 530hcx - I have seen mine lockup, but I think that may be caused by it being on the belt clip w/ jacket - multiple buttons get pushed at once. Haven't really tested this theory, it doesn't bother me that much, and doesn't happen all that often. Another issue I have seen is loss of audio - button beeps, radio audio. Again, I suspect buttons. Both are fixed with popping off the battery and re-starting the unit. Other than that, it works great!! I have FRS radios, and I now give one of those to my girlfreind while going shopping. No more missing people!!!
And I also have the Li-on battery pack, it lasts all day for me with the GPS left on with 1/2 screen brightness. (And if I forget to plug it in at night, it is still good until the morning!) I have the AA battery pack as backup, and I can charge the Li-on pack by itself, not attached to the GPSr. I have City Streets 2008 and Topo 2008, I loaded the east coast with room to spare on my 1GB card.
GSAK loads my caches for me, and it does hold 1000 before saying it is full. I could load a lot more if I convert them into custom POI's, but I haven't gotten that far yet. There does not seem to be an easy way (if at all) to load custom icons for the caches.
It has great sensitivity under the CT tree canopies, and gets plenty of satellites in the house too.
Overall, I would say this is a great unit, and I love it!