Well I would say yes because the soon-to-be 4 extra cases will be $16 by themselves so you can get them for $10, or less if you are playing the PS3 version and have left over money in your wallet as I did.
I have played both pre-order cases (Slip of Tongue and Naked City) and I would say each of them was about 60-90 minutes long, as long as any of the other major cases. Naked City had more action, was longer, and was harder to get a good rating on (no guide I got 4 stars on Tongue and 3 on Naked City.) Next case, an arson one, comes out next week and the final one, another vice one, comes out in late July.
Both DLC cases so far have 5 achievements/trophies for each, 4 hidden and 1 just for beating the case (which is a gold one on PS3 so 2 easy gold trophies.) I would imagine the next 2 DLC cases follow suit.
They are of course best played as you're going through the game so you get the XP during them, but can be played fine on replay mode, just don't get XP. In terms of the story, Naked City ties in with the main theme of the Vice desk while the traffic case does not. Naked City also properly introduces your old Traffic partner as Rusty's new homicide partner so if you thought that reunion was a bit awkward in the game, it was because this case had the proper introduction.