My 3G is still going strong. Somewhat battered sure (I've dropped it, had it in pocket with keys etc), and currently it requires the internal audio connector re-soldering after a botched opening-practice

But it's working fine. Best "classic" iPod they ever made, IMO.... no moving parts except for the lock switch and the HDD = fantastic, and very pretty backlight on the buttons
I finally upgraded to an iPod Touch after all these years, and don't regret it at all. It was a bit of expense, but worth it and still £30 less than what I paid for the 3rd gen back in the day. Not to forget that it's not just an iPod now.. it's a portable video player and the best handheld Wifi web browser I've ever used.
If you really can't justify the expense though, just get a replacement battery for the 3G and it'll have a new lease of life

It's still a great player, and ironically a few GB larger than the all new Touch so if that's all you want I see no reason not to squeeze every bit of life you can out of it. The batteries are dirt cheap, and easy enough to install. Popping open the iPod is a bit of a swearing contest the first time but you get there, and it's easy enough from there - just be careful with the audio connector inside, ease it out with a sharp knife or something by the plastic part, don't just pull on the ribbon as the solder joints on the connector WILL break!
I've literally dropped my 3G while it was running (so was I), and that's how I "opened" it for the first time, as it split apart on impact and the HDD detached from its connector and skidded across the road. I was sure it was a goner. The outer casing certainly gained a couple of dents and scratches, but I popped the HDD back in its place and clipped it all back together and away it went

*Very* well made, and TBH I don't think Apple make that standard of quality any more, although I can spot when they do (e.g. the iPod Touch, finally a worthy successor)