Tamron AF 20-40mm f/2.7-3.5 SP Aspherical IF

General
There's a lens that on paper looks promising. A full-frame, autofocus, (generous) wide angle lens. A zoom, in fact, that takes you from wide to almost normal. Its price is also very modest. So, is there a catch?

This is how it looks like. Clearly older design

Pros/Cons
+ very good optically, particularly stopped down (you would probably shoot a wide-angle at f/8 anyway)
+ Very decent construction quality, I think it will take a beating (although I didn't try!)
+ great value; you get what you pay for...

20mm on FX is really wide and can get you some spectacular cloudscapes

- ... a fact that can be repeated in a negative way: this lens is good but not great. The Nikkors or the Sigma 24mm prime easily outshine it.
- very susceptible to flare. Use the hood when in sunlight
- although not a deal breaker for wide-angle, not having f/2.8 @40mm is a minus (read final verdict, below)

Intended Users
Great for:

• FX users on a tight budget looking for a flexible wide-angle zoom.
• As a general landscape lens, it can be very useful (notice I don't make any qualitative remarks; I can't call it "excellent" or even "great"; simply "useful").
• casual, candid street photography.

40mm is close to "normal" territory in terms of focal length. Stopped-down, the lens can deliver good results

Not for:

• DX. It's kinda meaningless - just use the 18-55, and you'll get as good or better photos.
• Highly detailed texture, particularly without plenty of light. Wide-open, it's only adequate.
• Live Music photography. Although the range would've been decent (if you can be close to the stage anyway), f/3.5 @40mm, mediocre optical quality wide-open, as well as flare issues keep this lens from being a great live music lens.

Final Verdict
Let me clarify this, in case I was misunderstood: This is not a bad lens. On the contrary, it's great value; you get what you pay for, and perhaps even bit more (if you need a wide angle zoom and you can't afford anything better, this will serve you well). The problem with this lens is, ultimately, its scope. You see, when you pick a wide angle zoom such as this one, that goes all the way to 40mm, it starts posing issues of lineup. If you own a 50mm f/1.8 or f/1.4 lens in particular, you will feel the difference very intensely: The Tamron is not as good at 40mm and f/3.5 as the prime is at 50mm (even wide-open, not to mention @f/3.5). For some applications (such as daylight landscape shots) this might be irrelevant, and you might enjoy the flexibility of the zoom. But as the light goes down, carrying a 24mm and a 50mm prime becomes much more sense-making.