I too like Andrew saw the plume from the Comet SL9 fragment G hitting Jupiter, it was as he described. We had an impromptu viewing night at my place, half dozen scopes shoehorned into my backyard. As the explosion plume grew, we could see it in progressively smaller scopes till finally it was visible in a 4 inch reflector. Later, as the impact site rotated into view it looked as if a huge chunk of the planet was missing. Totally amazing.
Another memorable one was seeing for the first time, Comet Mcnaught, at midday. I aligned the argo navis on the Sun, then moved my 10 inch scope to where it indicated the comet to be. Sky was so bright I used sunglasses, but tere it was a hazy spot with a slight elongation. Seeing it from Lostock was excellent too
Another was at one of the earlier SPSP,s where I saw the "bright" part of CG4 with my own eyes through Peter Brobroff's 20 inch scope .
Another was Comet Hyakutake, from the dark skies of Gloucester, at 3 am. Its tail was twice as long as Mcnaughts. but much thinner.
Finally, recovering Comet Halley after perihelion on the same morning as it was first officialy recovered. It looked just like venus, but with a hazy mist about it. I was using 8x56 Binocs