These days you shouldn't buy a GPS unless it has a SiRF Star III chipset. The difference in sensitivity to other chipsets is very noticable. My own BT GPS (which doesn't have that chipset) is quite good, but it doesn't work behind certain types of glass, and in cities with narrow streets and tall buildings it can suddenly (incorrectly) direct my car nav. system to tell me to turn back..

GPS devices with the SiRF Star III chipset is so sensitive that the GPS can often be used indoors, and it has much less problems with city streets (or deep valleys). Edit: I believe that Holux you were looking at has this chipset. Other than that it should support NMEA 0183, most do.