Just thought I'd add my views on Operation: Anchorage. I installed the add-on pack while I was still halfway through the main game and the two add-on quests became available, so I thought I'd check them out. The set-up for Operation Anchorage seems a little silly but if you play it like I did as a sub quest during the main game, it's actually quite an enjoyable little diversion, and once you've beaten it (which won't take more than an evening) you're back into the wasteland with a couple of new toys. I can understand people buying this having finished the main story being disappointed by this add-on though, as it doesn't really add all that much and what it does add is nowhere near as good as the main game (as described elsewhere, it's a linear fps style mission so none of the depth and exploration possibilities as before).
I've not got stuck into the Pitt just yet but from what I've read it's more of your typical Fallout 3 goodness in an entirely new area, which sounds more like what a game expansion should have.
I'll conclude by saying that I think buying the two add-ons on disc is worthwhile if you haven't finished the game yet as Anchorage works as a sub quest/dungeon add-on, but if you have finished and want more to do then downloading the Pitt separately would be the better option.