Are Aquifers actually good? They sounded like a terrible thing to me, because Rivers seem more convenient.
Aquifers normally only take a few minutes to pierce if you know what you're doing. Single layer ones, and ones in frozen biomes are particularly swift, I believe a single layer aquifer in a cold climate can be pierced in about one minute if you make no designation errors. Multi-z aquifers take 4 or 5 minutes using pumps. Like many things in Dwarf Fortress, aquifers are not difficult per-se, it's just that you need to be fairly precise, if you only half understand what you're doing it's really easy to stuff things up so you end up with a system which consumes considerable time and resources and outputs dead dwarves. In that regard, they are like minecarts... you can do fun things with minecarts, but it's considerably easier to make a system that does not work at all but does a superb job of maiming and killing dwarves.
If I'm feeling not-lazy one day I might write the definite guide to raping, pillaging and utterly humiliating aquifers because the wiki article is not that good.
Putting aside the fun factor of conquering aquifers they give you limitless water from above and below. For example with an aquifer map, if you want to to tap water underneath, you just make an up/down stairs until you hit the aquifer, the miner then runs back down the stairs (dwarves outrun falling water, don't ask). If you want a greater volume of water flow, you punch the staircase clear through the aquifer and the miner exits through the top (water is also incapable of flushing dwarves down stairs), if you want even more flow, you can channel out a wider 'collection pit'. If you want water from above, a simple channel is all it takes, you can then pump water out. While a river is limited in the water it can output (and if you want more water per second, you have to store it), an aquifer is virtually unlimited, it's just a matter of opening up more tiles to get more water. If you pump water into a channelled out pool in the aquifer, then the pool gains the 'flowing water' attribute and will power water wheels automagically even if the pump is removed. You can dispose of water directly into aquifers which is helpful for drowning traps. For me the aquifer is so little bother it's worth having one just for the hospital water supply, but if you want to do some serious waterworks then aquifers are all sorts of good fun.
The final thing is when digging enemies finally come, the aquifer will be an equally big wart for them as it is for most players.