I'm not sure what stats a tank destroyer has in CoW 1.5 but in the normal one, its pretty good. Combined with a afew light tanks to fend off infantry, a tank destroyer is very good at breakthrough. I'm not sure about the stats in 1.5 but 'm pretty sure its probably good against light class armour in 1.5

High mobility is a big point.

If it actually had that... but in fact it is pretty slow.

Well, target type then. One is leg, the other is a vehicle. That can be an advantage.

But I do think there's a definate speed advantage at the next levels.

I evolved to 1.5 So I can't remember exactly.

That said, I do remember not finding the SPAT worth the cost. It's very situational. Fortunately it's much better in 1.5.

Besides the higher mobility of TDs..
>> AT's damage class is "Infantry"
>> Tankdestroyer belongs to damage class "Armored" and, unlike "real" tanks, has no HP penalty in urban terrain

Well compared to anti-tank, on the positive side:
Tank destroyers cost less manpower to produce.
Have different armour class.
Have 3 times more HP than anti-tank does.
Have a damage bonus of +50% on plains.
Are faster.
Have no manpower upkeep and less food upkeep.

On the negative side:
They cost slightly more goods, way more steel and oil.
Production takes almost twice as long (so you need to build them in advance and they can hardly ever function as a quick last resort).
Have an oil upkeep.

Things mentioned above mean it can do some things normal anti-tank can not.

It can effectively defend plains, anti-tank struggles here because of the lack of HP and the bonus tanks get in open plains but anti-tank guns do not. Because of their HP and power bonus in plains they also are not as bad agains infantry as normal anti-tank is.

As Restriko mentioned, it does not get the HP penalty all other armour gets in urban terrains. This means that, tanks are x 0,5 times les resilient and 0,5 times less powerful. But tank destroyers are only 0,5 times as strong.
That means that although combat takes longer in urban terrains, the difference between tank destroyers and tanks is actually the biggest in urban terrain. Think about a terrain type where the opposite would happen. Tanks and tank destroyers their power would be the same, but tank destroyers get 2x their HP (which is already bigger than that of a medium tank).

They are really good at what they are supposed tot do, defend. In the attack they are not much use until later on. Also, their power creep is amazing when compared to most other armour. It is the first land unit that can do more than 10 damage to armour. Being 10,5 on the defence in plains right on their first level. Medium tanks have an attack of 6,75 in open terrain on the same (first) level.

It can be a really good idea to have tank destroyers in your army. Especially in places that would otherwise be vulnerable to tank attacks and even more so if you are surrounded by advantageous terrain.
Since in the early game a lot of players tend to build light tanks a lot more than you really end up needing, I usually build tank destroyers before I build medium tanks. Depending on what my enemies are doing of course. Combined with your starting infantry this means that your defence is usually a lot higher than any enemies attack. Staying in your core provinces then means that you are not an easy target to rush or take out.