The savings to be made on the older glass on eBay is quite considerable. Even newer lenses can be had 2nd hand for a steal sometimes. I picked up a mint condition, boxed, 3 month old Tamron SP90 Di for £200, a saving of £140+. Similarly I picked up boxed, mint condition 24mm f/2.8D for £180. That lens still retails for £340 at Jessops!
Obviously with some lenses like the Ai/Ais you have no autofocus and no VR with the D lenses but, its not a feature you always need/want. For landscape, posed portraiture and still life, where the camera is tripod mounted, you want to turn the VR off anyway. The old lenses are also fabulously well made. Today's lenses feel kind of cheap in comparison but, I guess that's the price you pay for lightweight lenses over solid construction.
One of my favourite lenses is the Nikkor 28mm F2/8 Ai-s. Stunningly sharp lens, manual focus and manual aperture. No metering either on my D90. Still absolutely love it. There is something about the feel of those old lenses that you just don't get with new glass - although if someone wanted to swap me for for a new 70-200mm f/2.8 VRII, I'd consider it lol