I have thoroughly enjoyed the extensive life of my i9000, having flashed an unimaginable amount of ROM and kernel setups, tweaking it in every way I could figure out how to. Now that the i9000 ICS ROMs have reached a state usable as a daily phone, I must admit that it feels like I have a new phone again. ICS is a wonderful improvement.
That being said, a good friend of mine recently purchased a Galaxy Nexus. Actually, he purchased it before I had flashed ICS onto my phone. I thought the phone was a definite improvement over my i9000, but to be honest, it was just ICS. Our phones can play the same games, do the same things, and our pretty much equal. Sure, his dual-core processor gives him a bit more speed doing things like installing apps and such, but my i9000 is unbelievably responsive after upgrading to ICS. Before it, I was going to upgrade. Now that I'm using ICS, I have absolutely zero reason to upgrade. None at all. My phone works just as well as a new one.
Now if I had never discovered XDA and was still using the stock Gingerbread firmware, I would definitely be buying a new phone. Thank god for our wonderful devs!