I have some spare points, and ME2 DLC seemed like one of the best ways to spend those points. I am mainly considering Kasumi and or the shadow broker at the moment. 
 
How worthwhile are the three 500+ point content packs?

Go with shadow broker.  Picked all the ME2 DLC when it was on sale after Christmas and its definitely the best.  If you're into ME2, you'll probably like them all

Don't buy any of the packs that are just giving weapons and armor. There really are only 3 DLC packs, and two of them are somewhat small, those being Kasumi and Overlord.

They were on sale a couple weeks ago at 200mp a piece. 
   
Thats when i got all three, Overlord is really good i just played through that and it lasted me about 2hrs, and has a cool story. Kasumi is pretty good not as good as Overlord, but it's an extra character and her loyalty mission is around a hour long (on the ship you only interact with her like Zaeed, meaning you can't have a full conversation with her). I haven't played Shadow Broker yet but that is definitely the one to buy from what i've heard. 
 
I'd say Overlord and Shadow Broker are the best, and if you have money get Kasumi if not you won't be missing out on much besides a pretty good SMG.

The Kasumi SMG is pretty damned good. Love that thing. But as a mission it's a bit on the short side. Shadow Broker is definitely worth the price. Overlord ... it's okay. I know some people loved it, but the story didn't do much for me. 
 
The rest of the DLC isn't worth what they're charging.

I've bought all Mass Effect 2 DLC, even the weapon and armor packs and the ones I could recommend are these:
 
Shadow Broker is probably the only of the episodic content I found to be really good. Even if Overlord had some good moments, it just didn't add anything to the story of my Shepard.
 
Kasumi's stolen memory is of course a given if you want all squad mates and she does have some interesting skills with her back stab move (you also get a pretty good SMG from it so that's pretty good as well).
 
 Also the Firepower Pack is a good weapon pack because it gives you an insanely over powered, single shot, assault rifle and a competent heavy pistol with quite the punch. There comes a Geth shotgun as well but I never found good use for it.

I just completed ME2 including the three mission-based DLC packs. I bought them all during the Christmas sale before I started my first playthrough, so my experience may be different than the one you would have. As others have said, Shadow Broker is by far the best and is especially great if you're into the lore of the ME universe. It brings in Liara from the first game and hopefully sets her up for some good stuff in the next sequel. 
 
I like the Kasumi character, and I found the mission fun. She was one of the first characters I picked up. Recruiting her and doing her loyalty mission was, for me, integrated nicely into the game as a whole. Aside from a lack of things for her to say during later mission cut scenes, she was like any other character. That said, if you're just going to load up the game after the ending and play through her mission it probably won't be worth it. I would only pick this one up now if you're going to start a new run of the game.
 
Overlord was fun, but it was a very standalone mission. Unlike Shadow Broker or Kasumi it doesn't really tie in much with the rest of the game. It's fun to play through, but you wouldn't be missing much by skipping it. I say wait for it to go on sale again unless you're dying for more ME2 right now.

I would rate the packs I bought in the following order:

1. Shadow Broker - best content from an enjoyment perspective, from a looks perspective and for what it adds to the overall universe

2. Kasumi - she's a cool character, her mission is fun and she has great abilities!

3. Firepower pack - I've never bought DLC weapons before, and I don't plan on making a habit of it, but for the price the new assault rifle added more enjoyment than Overlord

4. Overlord - pretty environments, fun combat encounters and good length but it doesn't have much in the way of character interaction, it's mostly combat.

Well, Shadow Broker is story stuff so you should get that. If you however want to make the combat much more entertaining then you should by Firepower Pack. Kasumi is a great character who I had in my squad through my journey after the release. Overlord was also pretty good. But yeah, the other ones are better. 

EA is having a sale on the XBLM next week. You might want to hold out. If ME2 stuff gets cheap you might be able to pick them all up instead of having to choose.

Goddam I think you guys might have made me buy the Firepower Pack o_O