I bought my S3 outright, so I still have an upgrade available on my line. With the launch of the S4, I've considered a few things...
1. Use my upgrade and get an S4. To do this, I'll have to release my unlimited plan. With my previous job I traveled a lot and used quite a bit of data, but honestly within the last year I haven't gone over 4 GB once, so it's not like switching to a limited data plan would kill me...but there's still a part of me that wants to hold onto unlimited data as long as Verizon will let me.
2. Switch carriers. As my Verizon contract is up, I've also floated the idea of switching to T-Mobile or AT&T. I really like T-Mobile's new plans, but unfortunately they just don't have the coverage I need in all the areas I frequently visit. AT&T I just don't see being enough of an "upgrade" in any way to leave Verizon for them. None the less, it's still an option on the table.
3. Buy the S4 outright as I did my S3. Not going to happen -- My S3 runs too well to spend that kind of money on an upgrade. At that price, I'll wait.
4. Just keep on keepin' on with my 'ol S3. Honestly, this is probably what I'm going to do for now. At the very least, I won't buy an S4 with any carrier until the bootloader is confirmed unlocked, but besides that, my S3 runs great and (knock on wood) is still in great shape, so besides my general geek lust for all things "new", I'm not really clamoring for an upgrade...yet.
So, to answer the OP's question...yes, I'm happy with it. Will I wait for the S5? I'm really not sure about that...chances are I'll cave before that and buy *something* new, whether it be an S4, HTC device, Nexus, (etc), before the S5 arrives -- but that has much more to do with my own addiction to technology than it does with my satisfaction with the S3.
Given how locked down the bootloaders are becoming, I'm actually leaning a lot more towards waiting to see what the next Nexus phone has in store for us, even though I know I'll almost definitely have to switch carriers to get it.