If you can find it for a dollar, or laying in a gutter, AND you have a high tolerance for pain, or are an obsessive collector of comic-to-movie stuff. Then go for it. Otherwise you will likely end up feeling ripped off.

i bought it for $3 on clearance at Circuit City. i got ripped off.

Don't over-pay for
The Invincible Iron Man
, buy it if you can find it for cheap.
On the other hand, so far there are no references made to this film in the Marvel Ultimate Animated franchise. So it isn't exactly required to be owned if you're going for story completion, and it's likely you'll only watch it once. I guess it sorts of informs the Iron Man we see in Ultimate Avengers 1 and 2, I mean it's a proper prequel for the character, origin story and all.
A better viewing experience for newbies might be Iron Man first, then Ultimate Avengers, then Avengers 2. That way you're going from exceedingly mediocre, to good, to great.
I also really enjoyed Dr. Strange, glad I bought instead of rented. I was ready to start renting these after the somewhat disappointing Iron Man, but it's apparent now that we're gonna get something very different depending on who the focal characters/franchises are, despite it being mostly the same people behind these films each time.

i really like all the marvel animated movies. I say it's worth buying.

Right now Best Buy has it for $4, so I'd say it's worth that.
drew
Cid Highwind: This is Man Talk.
-FF Advent Childeren

It depends. If you liked the Ultimate Avengers movies then this one is worthwhile. I didn't enjoy it as much as UA, but bought it anyway. A little heavy on the mysticism schlock, light on the intellectual. It does not follow either classic or ultimate versions of Iron Man.
What's the point of being cool if you can't wear a sombrero?Hobbes