I've had mine for about a year and I sent it in for the 4G upgrade.
It is faster, but not by huge amounts. If I would've had to pay for the upgrade, it wouldn't have been worth it.
Battery life has certainly taken a toll and the device doesn't seem to run as solid as before - more force closes on the browser and weird non-connectivity problems (!).
As long as I reboot the device every day or so, it runs smoother, but what a pain.
Not really, in my opinion.