So I've been trying to find time to sit down and give detailed thoughts on 5100s since I've become sort of a guru on them lately, but since I don't have time I figured I'd at least start with this quick rundown...
Pros: Affordability, compatibility, life span, different lengths available, ride quality on pavement
Cons: Steel bodies, limited cooling capacity, not rebuildable, can be paired with coils which their dampening is poorly matched
2nd/3rd Gen (front) part number differences: The 3rd Gen 5100 shocks are shorter with less overall travel and have 5 spring seat settings specifically for factory 3rd Gen coils. The 2nd Gen 5100s are longer with more overall travel and 4 spring seat settings specifically for stock 2nd Gen coils. They have different valving because of the difference in spring rates between 2nd and 3rd Gen factory springs.
What this means: If you are going to run 5100s on your factory 3rd Gen coils, I would recommend the 3rd Gen 5100s. If you are going to run aftermarket springs and upper control arms, I would run the 2nd Gen 5100s. Why? More travel is better, and the 2nd Gen 5100s will be valved more appropriately for most aftermarket coils that fall in the 590-620lb spring rate range.
How much more travel do 2nd Gen (front) 5100s have? More than 0.5" additional compressed (uptravel) and over 0.25" longer extended (downtravel) than the 3rd Gen version. If you're not that familiar with suspension then those figures may sound like nothing, but if you knew how much money and effort some people invest in their setups to gain 1" of wheel travel on an IFS rig you'd probably vomit.
What about the rear? Bilstein's "B8" part numbers for the rear 5100s are the same for 2nd and 3rd Gens and only accommodate about 1.5" of lift, so they will work with most add-a-leafs with or without overloads, and the "b" word that ends with "locks" (but please, don't run blocks). If you run a full aftermarket replacement leaf pack such as the Dakars, Bilstein also makes a "B110" rear shock that is crazy long and compatible with just about any heavy duty leaf pack combination out there, but in order to run them you will need extended brake lines or at the very least brake line extension brackets and a little creativity.
Who am I to talk? Just a dude who wheels with a combined 125,000 miles on four different sets of various 5100s on two different generations of Tacomas and six different spring combinations who happens to actually use the shit out of them on trucks that were absolutely not pavement princesses.
I could go into sickening detail about my experiences with 5100s, but the bottom line is that they are a great entry-level shock option that is superior to OEM in every way. The adjustability makes them versatile, as does their ability to be paired with OEM, Eibach, OME, and other "stock replacement" coil springs to get the ride height you want with or without preloading them and losing suspension travel and ride comfort.
Likewise, I could go into coma-inducing drivel about how 5100s are inferior in almost every way than virtually any coilover or reservoir shock on the market, including budget-minded 2.0s, due to improved cooling, dampening, valving, adjustability, spring options, and the list goes on and on. At the end of the day, you should always get the best that you can afford if you use your truck off-road often, but for those on a very limited budget the 5100s are arguably the best option. They are certainly far superior to any spacer lift, less expensive and very similar to Eibach's newer offerings, and better overall than OME and just about any other lesser known OEM-style shocks/struts on the market.