I'm using ATC Poly-Fair on all vinylester surfaces. This is great stuff, even at $200 for a 5-gallon can. It's light (18 lbs for 5 gallons, and I estimate 30 gallons for the entire boat) and easy to apply, yet doesn't sag at all. It mixes with MEKP, so cure times are really flexible, and even with low ratios that allow a long working time, it cures quickly and can be sanded in a couple of hours. The MEKP causes it to change from bright pink to more of a silly-putty color, so you can tell when it's completely mixed in good light.
For epoxy surfaces, I'm using several approaches. I purchased some of the new System Three QuickFair epoxy fairing compound. It's also easy to use, has great consistency (like peanut butter), and the two parts are purple and white so you have a good idea when it's adequately mixed. It cures relatively rapidly and sands very easily, and each batch has exactly the same consistency and sanding properties as the last. The big problem is cost: about $150 for 1.5 gallons. I don't have too many epoxy areas to fair, but that's still really expensive, especially considering that there's probably less than a gallon of actual resin in there.
The next tactic is good old hand mixing. I found a G.E. electric mixer that attaches to a stand to make a poor-man's KitchenAid. (Good thing, too, since when she saw it, Tanya asked if I had gotten the mixer from the actual kitchen.) The cake batter beaters are good at mixing resin and fillers. I squat on my stool in front of my folding tray table, and dump equal parts silica and micro for filleting, or mostly micro for fairing (or all micro for fairing flat, horizontal surfaces). It's tedious, and I have to wear my respirator, but you can't beat the price. The cheapest resins you can buy are around $40 a gallon. Microballoons are around $15 a gallon. If you are lucky you can increase your volume by about 70%, so 1.7 gallons of homemade fairing mix is around $50, a third the cost of the premixed stuff. Just remember to eject the mixer blades and dump them into the can of vinegar before they cure, or you have a mess to clean up next time.
I have used microballons, Q-cells, and microspheres for fairing and filleting. To be honest, I can't really tell the difference between Q-cells and microspheres. They all sand well. Supposedly the Q-cells are stronger, so I used almost all of the bucket for filleting. The microspheres are finer and don't clump as much as the microballoons. Whenever I mix a batch of microballons, I have to mash the clumps with my tongue depressor, even if I'm mixing with the electric mixer. Next boat, I'm going to avoid microballoons completely, and I might even toss the rest of the 5-gallon pail when this boat is complete.
At one time I believed that a thick resin would be best for fairing, since it would reach a non-sagging consistency with less filler. In fact, the opposite is true. Thick resins reach "peak" much sooner than thin resins. I define peak as the point at which you can add no more filler, or risk having the resin be too dry and non-spreadable. I tried some USCI thick resin and fortunately only bought 2 gallons since it's pretty much useless for anything but coating bare wood (they did warn me, to their credit). I'm going back to their thin resin, which is one of the least viscous I've used.
My main fairing tool is the Makita 7" sander/polisher. Once you learn to use it (and learning includes equipping it with a soft backing pad, intended for buffing), you won't look back. It took me a while to process the fact that to make fiberglass smooth requires an order of magnitude more power and grit than wood. It's basically of the same hardness as metal. On my first few small boats, I spent hours with random orbital sanders, and never achieved a really good finish because I was making very smooth little hills and valleys. Now I use the circular sander most of the time, the belt sander occasionally on flat horizontal surfaces, and the r/o for finish sanding and for edges and concave curves where the circular sander would gouge. Mostly I use 50-grit discs, and 60-grit for the r/o. I would use 50 grit in the orbital but it's too hard to find and too expensive when you do find it. Buy discs in bulk, and always have plenty around. I can wear out a 50-grit disc in a half hour of continuous sanding. I can wear it out in about 8 seconds if I accidentally touch a surface that isn't completely cured, or has amine blush, or is waxed polyester.
I also just bought a Bosch 3 amp random orbital sander. My first impression is very positive because it's very powerful and spins at high RPM. Much heavier than my little Makita, but sands much faster.
Even after several weeks of fairing the hull, I'm only starting to see the light at the end of the tunnel, and that's just for the starboard side. Next boat, I'm unquestionably using only unidirectionals, because no matter how smooth your foam is, the overlaps will kill you every time. I also should have tried the technique of using a notched trowel, particularly for the little valleys adjacent to the overlaps, but it's probably too late now to be experimenting with a new method.