A definite bargain in my opinion, you can get a lot of mileage out of the X games if you put in a bit of effort, and these latest ones are quite beautiful. However, it's largely not worth having both titles as they're extremely similar.
The retail key for Terran Conflict activates in Steam, by the way. I picked up a copy of X3: TC on eBay about two months ago for $5 delivered, and just whacked the key into Steam. Could help someone willing to save a few pennies by waiting a few days.
I'm not sure if Impulse will provide a CD key, but I would expect them to, and it should work in Steam just fine.
Also, check out the modding community!