Tamron SP 500 mm f/ 8 Adaptall-2 model 55B Lens

Light to carry around
Quality build
Super smooth focus ring
Can focus beyond infinity
Rear filter system 30.5 mm filters are more practical to carry around than 82 mm ones
Virtually no chromatic aberration
Can close focus so you can make interesting macro shots wit it but there are more practical tools for that (but not always)
Detachable but integrated hood
With Tamron 2X flat-field tele converter you can get 1000 mm lens on FF body(you can stack two of them)
Adaptall-2 mount with proper adapter it will fit almost to any 35 mm camera system

Weaknesses:

Tripod (usually it will cost more than lens and it will weight more) is a must also mirror lockup function and remote trigger 
Not practical for sport or any fast action photo I'm not saying that it can't be done but every occasion asks for proper lens selection
Hard to clean lens front element 
Focus confirmation chip is not working in most situations cause of f8
Shallow DOF
Somewhat disturbing bokeh (but can be controlled with frame planning)

Comments:

This is not a lens for beginner it is essential to know how to focus lens without any electronic assistance on a DSLR. You can always obtain split screen with micro-prisms but that can produce unwanted effects with modern optics.
You will find a lot of people complaining about this and that. You will find people compare this lens 200-300 USD with 2000 -3000 USD lens from Canon L series (is that fair)
This is what you can expect:
If you are using it in a cold weather let the lens cool down to ambient temperature because it will shift focus a bit it is a lot of glass and metal.
This is not a lens that you will use a lot but it is a light one and you can afford to carry it around and use it if weather and opportunity presents itself.
You can use it for taking a moon pictures but for that you will need a lot of patience practice and perfect weather and don't forget moon is a fas moving object (and we are not talking about telescope).
Lens is nice for taking pictures of wildlife bird nests and other non suspecting animals, you will not catch hummingbird in flight unless you can predict it's path.
I saw some nice pictures of birds in flight but they are rear
All in all it is a nice lens in its group it is not fair to compare it with any other than other catadioptric lenses but in certain condition it can outperform or match much better and expensive optics.
I'm waiting on good opportunity to obtain 55BB model and to compare them one to another.
Ahh yes  I almost forgot BOKEH and dough-nut effect some like it some don't. As I said before it is not a lens for beginners,  dough-nut effect can be avoided (clean background and no doted light sources in background) if picture is planned and executed with care, this is the rule for all catadioptric lenses not just for Tamron 55B
There are some old catadioptric Vivitar series 1 and CZJ lenses out there that are known to be better but they are all with bigger much bigger price tag.
If you plan to obtain one make sure that it has all the trimmings: front cap, tripod mount, neutral filter is essential as it is a part of lens formula (there are some other filters like ND2 ND4 and skylight), it must have hood.