Much like Bob12dozen, I started with the Samsung Focus w/ WP7.5.
I saw the GeoCache Live app listed on the Marketplace back in Oct. but did not really get into GeoCaching until late Nov.
I loved it and as a Land Surveyor, found myself pulling it up every new job site to see what was near me.
I soon discovered I was lucky if I got 2 or 3 caches (5 or 6 if C&D) on a good day, before I would have to camp out on my car charger.
I few months ago I bought the Magellan eXplorist GC and now that is my main unit, but I still us my phone.
Just 2 days ago I upgraded to the Nokia Lumia 920. Still the WP7 app.
So here is my Short Pro/Cons:
Pro: Hand Held GPS is great for longer searches and harder one too. You will get tighter direction, particularly if in an area where Cell signal will crap out or get weak.
Con: You have to know where you are going to look ahead of time, whether it's syncing up and individually d/l the caches to it or Running a Pocket Query the day before.
WinPhone Pro: On the Spur, look where you are searches. Post right as you Find or DNF! Able to get up to date Logs and Photos.
Con: Will drain you battery, Bad Cell areas will make it harder to find some caches.
*I have a coworker who has the GeoCache Live on his iPhone 5 (cost $10 w/ no perk or boost to membership) Little smoother use of the app, and his can post to social network sights as well, not an option for the WP.
Con: The WP app has a Save search option, but there does not sem to be a way to save or upload these to GeoCaching.com to add to queries or watchlist. Also, the Selecting of these seach's is a bit broken at times (always jump to the 1st list and not any other.)
I'm hoping w/ the WP8 they will make a better app, not sure if I want to pay $10 or it, maybe $5, but if they want $10 I think they should at least toss 1 month of Premium.