I bought the game after playing trial for 9 months, and kept my trial account, which I still keep playing sometimes 'cause I love my chars.
The Pros of upgrading... you can group with friends without begging strangers to do it for you. You can actually talk to people. There's a lot more to do and see. You can use the Auction House, you can explore places without running for your life and dying all the time

. There are a lot of interesting things that you never get to experience as f2p. You don't get bored waiting for BG queues to pop after you've done just about everything you can. And flying is just spectacular

.
The Cons... it costs money. At 20 you know what to do, how to gear. After that, things get more complicated. Levelling can get incredibly boring. And questing is a lot less challenging than I'd hoped, which adds to the boring.
Very subjectively, I do enjoy the full account and still have an enormous lot to see and do, but I don't enjoy it
that much more. I still prefer the 20-24 bracket. I created lvl 20s that stick to f2p rules because I want to be able to group and do full account stuff without having unfair advantages in BGs. F2P has it's own community, and I haven't found anything that comes close in the full game, despite guilds and all.