I currently have a Galaxy S4 which I am pretty happy with. Is the S5 enough of an upgrade to warrant the purchase?
I'd wait a bit longer also. I upgraded from an S3 and could have probably waited for the next big thing.
I previously had an S4 and upgraded to an S5. It all boils down to you whether you think it's worth upgrading from an S4 to an S5. I suggest going to your local mobile store and play with the S5 that is on display. Personally, it was worth it for me. Battery life lasts way much longer than on my S4 (I probably had a defective battery on the S4)