I am successfully using my G1’s 3G connection via my laptop. I followed the instructions on Graham Stewart’s website. I didn’t have instant success so googled for more info. I needed to turn go into Firefox’s config settings by putting about:config into the address bar. Ignoring the warning, I changed the setting for network.proxy.socks_remote_dns from false to true, then restarted the browser. Success!
I visited Speedtest.net to see what sort of speeds I was getting. Here are the results:

Not too bad!
In comparison, here is the connection speed via Virgin’s ADSL, not their cable broadband service:

Obviously my ADSL speeds are much faster, although the upload speed is only 2.5 times faster. The download speed is 20 times faster.
I’m pretty happy with my G1 so far. It came with a crap pair of headphones though, so on eBay I just purchased a 3 in 1 USB and 3.5mm adaptor. The adaptor has a 3.5mm headphone socket and two mini-usb ports. This means I can charge it whilst listening to audio. The adaptor was listed on eBay for £10.49 but I made an offer of £8 and it was accepted almost immediately.
I’ll report back if the adaptor doesn’t live up to expectations.