For every 10 "good" reviews you hear, there are 100 other people that have no idea that they have front end issues either developing, or fully developed and causing problems that they have no idea stem from the spacer lift itself.
There are more different ideas on what we call off-roading than there are lift kit companies out there selling parts. One of the major flaws is the fact that most, like mentioned already here, are not for decent off road use.
One design draw back with the Tundra's / Tacoma's is that the front axles spin when not in 4wd. Throw a lift / leveling spacer in, and your front boots hate you a lot sooner. If the truck stays on street 99% of time, then the issue is not as prevalent (this applies to 2wd trucks too). The more the truck is used on rough asphalt roads, dirt roads or 4wd situations when you need it most you will have issues as the spacer will let the shock droop out past safe factory points.
Spacer lifts go against everything for a truck that gets used and abused. We do not sell spacer lifts here at SDHQ for all of the above reasons and many more. If a customer is worried about budget, we talk them into a 5100 Bilstein. Factory stroke is kept, keeping the CVs and boots happier for longer than a spacer kit could ever provide.
Some people complain about the ride of the 5100's saying that it is "stiffer" than stock and that the ride quality has diminished. This is not entirely true. The ride quality remains similar, as the factory coil spring is kept so the rate is the same. The issue comes from the top clip setting to "get the most out of the shock" lift wise. This limits the down travel from ride height to allow the wheel to fall into a hole and absorb the impact. The shock "tops out" faster than stock because you are closer to the pistons internal stop sitting at ride height. That internal stop is there for a reason, just like the stock Toyota shock, to stop over articulation of the factory suspension components (I.E. CV's and UCA).
All in all, there is no spacer that can do the job of the correct length shock. As you save pennies, never rule out the the true coilover option. We have hundreds of customers every year that are ecstatic in just the daily driver performance of a 2.5" internal resi coil, and happy that they didn't waste their time on a spacer lift.