Vivitar Series I 90/2.5 macro, made by Tokina, AKA the "Bokina"
How to review a "legend"? Well, even a legendary lens is still just a photographic tool.
As a macro
On its own the lens only focuses down to 1:2. There is a matched 1:1 macro adapter and I think these always came as a pair, but these days the adapter is somewhat harder to find than the lens. It is mounted between lens and camera, like a TC. With the adapter in place the lens now focuses down to 1:1, but its longest focus is reduced all the way to 1:2, so this is not much different than using a lens on extension. This is an inconvenience with the macro subjects I typically shoot, as I often range back and forth across that 1:2 line.
The lens is sharp, as any macro should be. I also have the Sigma 70/2.8, renowned for its sharpness, and I'd say the Bokina is in the same league. I prefer the Bokina's color rendition -- it is warmer and less saturated than the Sigma, very natural looking. The quality of the bokeh is the reason for this lens's reputation and it does not disappoint.
As a general purpose lens
By specification and with its bokeh this should be a fine portrait lens, if you like sharp portraits, but I haven't really put it to the test here. It's a fine general-purpose lens but with one huge caveat -- it is massively prone to flare. I'm not talking about sun-in-the-frame shots, just shots with indirect backlighting where you wouldn't necessarily think to look for it, and which other lenses handle without a problem. The Bokina just fogs over. Even a deep hood isn't enough to prevent all such problems, and keep in mind that this is a macro lens, with a recessed front element. This is the lens's Achilles heel.
Build quality
Many lenses are described as "built like a tank". On that scale the Bokina is built like a nuclear bunker. It makes a Pentax K-series lens seem a bit flimsy by comparison. The barrel is massively thick, with a high-gloss finish. The lens weighs about 640g and the adapter another 250g or so. The adapter has a tripod ring, very sensible for such a heavy lens. The focusing action is smooth and strongly damped, with the long throw you'd expect from a macro lens.
Early in my ownership of the lens, I put a TC on it, and it got stuck. The aperture coupling levers got crossed up. In the process of getting it unstuck, I may have bent something; at any rate it is now all too easy for this to happen, especially with the 1:1 adapter. I'll have to see if this is fixable; for now I've stopped using the adapter, or anything with an aperture coupling. I'm certainly not going to attach the lens to an uncrippled mount camera; if that were to get stuck it could be a real problem.
Summary
The lens deserves its reputation as "legendary". It also has some significant quirks, at least my copy does. I expect I'll keep it forever, both for its qualities and also because I wouldn't get a great price selling it given its issues with the coupling.