We use Union Ink, and print with both their suede and puff inks (actually, we use their additives and mix with whatever color ink is needed for the job). Puff is great -- you can go from just a slight raise (we use this for printing can coolers and some dark substrate jobs where we need good coverage with one coat) to a real puff effect. We haven't had a lot of call for the suede effect, but I personally like it -- the finished print has a suede look and feel to it. Both of these are best done on 110 or 80-count mesh, but often we have to use 155's and it works fine. Neither very fine lines nor large areas of ink are suited to these effects, but most fonts would be.
The gel ink can be used a number of different ways for different effects, from a glossy over coat to a water drop effect to a 3-D effect. Union's website has instructions for using all their inks, and I would assume other ink vendors do as well. A quart of the puff or suede additive is not very expensive, and will last a long time, since you will be mixing small quantities of it with standard plastisol ink colors. Start out by experimenting, mixing up small quantities in a deli one-pounder (like potato salad comes in) and playing with it until you get the effect you like. Be sure to keep track of the percentages you mix, and write them down and attach to your trial pieces, so you can reproduce the same effect later.