I used a Garmin 12XL for many years and it proved very reliable and 
reasonably tough (although the external antenna connection corroded badly, 
but I never used it anyway).
 I have had occasion to use a few different Garmin 76 series units (similar 
specs but are a lot lighter and easier on battery consumption) but was very 
unimpressed. They typically performed MUCH worse than the 12XL in direct 
comparions of time to first fix and in hanging onto the satellites. I 
wouldn't recommend them to anyone.
 But now I use a Garmin 60CX, which is a relatively new device which uses the 
SIRF III chipset. It performs so much better than the 12XL that I just can't 
see using the 12XL again. It is vastly better at finding satellites and 
getting fixes in marginal reception areas. I can usually get good fixes from 
way inside houses and other structures, even aeroplanes, and rarely lose a 
useful signal even in deep forest. The newer chipset is much more effective 
for field use, as I have found out also when using Bluetooth units of both 
types.
 The above applies to self-contained, hand-held units. But a much cheaper 
option, if you already own a suitable PDA, is to get a Bluetooth unit. These 
are very much smaller because they have no display, but use a Bluetooth 
radio link to a PDA (or laptop, if you like). Again, the Sirf III models 
work much better than the older chipsets. The problem with using a Bluetooth 
unit and a PDA is that now you have two devices which need batteries, but at 
least they don't have to be connected together with wires. Also, PDAs go 
through batteries much faster than modern GPS units, so if you plan to use 
it for extended periods, then a self-contained GPS is a much better option. 
I have had very good success with the i.Trek M3, which is the same as the 
GlobalSat BT 339 Bluetooth GPS.
 Yet another option, if you already have a suitable PDA, is a Compact  Flash 
GPS unit. This avoids the two devices problem that Bluetooth units create, 
but it means the PDA drains its batteries even faster. I have had good 
success with the GlobalSat BC 337.
Cheers, Chris.
Chris Corben
www.hoarybat.com
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www.birding-aus.org
birding-aus.blogspot.com
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