Is there any way of telling if or how many member only caches are in my area?
If you look up your area on the website they will show you where they are (minus exact coords) but will give you no other info on them. You can tell them in the list by the premium member icon. If there is another way I don't know yet.
We started out with the free app ourselves and went out with friends who had just started. After one night we bought the app. Since then we paid for the premium membership as well because we felt we were missing out on caches. We were definitely hooked after one night of caching. We just passed 100 last night and have only been at it for 2 months now. We are more urban cachers so we don't have a problem with the GPS. We have both the iPhone 4 and 3GS and both work great. As far as battery life, sometimes we trade off or we just use the charger in the car if we are driving around to a bunch, but mostly if you play with other settings on the phone (ie. brightness, etc.) you can help your battery last longer, not just geocaching but even in everyday usage.
We love geocaching and look forward to hopefully meeting some cachers in our area soon. It's a lot of fun and something you can do with the whole family.