I notice that a lot of the discussion I see about this is in regards to actually gaining XP from Down the Rabbit Hole (DtRH), but I think it has more to offer than that.
DtRH offers more than just some savings on XP, and I think the important thing to realize here is that you don't need to have a net positive XP gain, or even net 0. I'm of the opinion that this card is amazingly good even when it costs you XP. That might seem counter intuitive, but I prefer to look at this as acceleration for your deck's power at the start of the campaign. If it saves you XP on (for example) the first three scenarios, and you don't go negative until scenario 5 or 6, that means you're above the XP curve on 5 of the 8 scenarios, and only below for the last 2 or 3.
I think this is an amazing advantage of DtRH, since Arkham is a game where a lot of the difficulty is in the early scenarios, before your deck has all of the cards that it needs, and thus where you have more limited options. Additionally, early game XP means you're better equipped to take all the XP from those early scenarios, which means you indirectly get even more XP from DtRH. This means you can snowball into the late game with XP for your entire party, and with several good scenario resolutions under your belt.
This is opposed to the late game, where you probably already have a solid deck, and so can afford to take a penalty on the cards you purchase because you're going to feel that a lot less. Another way to look at this is that getting +1 or +2 XP when you only have ~10-20 XP is a %10 increase, where as in the late game where you can have 60+ XP, the penalty of +1 XP per card is just a lower percent of that.
In the end, I see this card as not being too dissimilar to early promoting in Fire Emblem. You're trading your potential late game power for an early game advantage. I think in Arkham (as in the Fire Emblem games I've played) that's a trade that will work out better for you in the long run.