How good is infravision
 

On the one hand, it says “eliminates vision penalties”, but on the other hand it says stuff about themperature differences.

The way I understand this:

Infravision is as good as daylight, for the purposes of combat and scouting and finding traps and tracking. Air is a different temperature from walls and ceilings and skeletons and weapons and plants.

The only downsides are:
a) you can’t read;
b) you can’t tell colors;
c) you can’t see some stuff such as Simple Illusion;
d) you *might not* see an earth elemental hidden against a rock wall.

On the other hand, there are no real bonus upsides (examples):
a) you don’t get to see invisible stuff;
b) it won’t help defeat stealth beyond the darkness penalty;
c) it won’t tell you whether something is alive or dead;
d) it won’t help with tracking.

It *would* tell you which of three stoves was recently on, because that’s just super obvious.

Is this the intended effect?

Re: How good is infravision
 

I would say that DF prefers to make abilities straightforward to deal with, so the default treatment is just those effects, but a GM who wants to go to the effort can apply more effects (both positive and negative).

Re: How good is infravision
 

Yep. The only thing that officially matters is "Eliminates darkness penalties completely when dealing with things that aren't thermalized – or that are, but that are also within two yards of a heat source." It's really just a way of saying that you'll ignore darkness in a fight unless the GM feels like saying you can't. Whereas Dark Vision is really just a way of saying that you'll ignore darkness, period.