I had a 300 AF-I and it was fantastic. Fast focusing, sharp as a tack and build like a brick. In fact the AF-I has more ED elements than the newer AF-S model. The downside is it is very heavy. I had both the 300 and 400 and only sold them because I got a deal on 200-400 that was too good to pass up.
Serviceability is a question. Several years ago I called Nikon and also talked to them in person at the El Segundo service center and they wouldn't give me a straight answer short of sending a lens in for service. What I can tell you for sure is Nikon's policy is to guarantee spare parts for 10 years after the end of manufacturing, and the 10 years is up. Of course there is no guarantee they don't have parts either. If Nikon doesn't have the parts I'm sure someone out there like KEH does.
For me, I wouldn't hesitate to buy one (300, 400, 500 and/or 600), I think they are the best value out there for Nikon super-teles.
I passed on a couple around $1300 because of the weight advantage by the AFS 300mm 4.0, which is plenty sharp and still a quick focuser (advantage to AFI with TCs). Gotta find the AFI priced well though, if it creeps into the 2000s like most eBay ones- I'd just go to the AF-S 1 to avoid part finding later if neccecery, and it's faster AF and a little lighter.
