I've been using an HTC HD2 for the past 2 years and it has been great. I use a Power Monkey for extra battery and download all the caches onto Memory Map, before setting out. I do not use a geocaching program in case it breaches the GC TOU, but if I did I would use GeoScout from Woodpecker software. Apparently it is brilliant and is a true all-in-one application.
The only problem I have found with using a smartphone is the issue of static navigation. The HD2 suffers from this, as did my Nokia N95. Having said that, using the hint usually gets you to the right spot so it is not a major issue.