This card is really fun and really good. Experience is immensely powerful in this game, and a multiplayer group that Delves aggressively can easily get about twice as much of it as a group that does not. A few tips for using Delve correctly:
1) With only a few exceptions (The Gathering and Extracurricular Activity come to mind), you don't want to Delve in the middle of the scenario. Even if there seems to be a quiet moment, Delving will rapidly make it unquiet, and this is no good. Rather, you want to Delve almost at the end of the scenario, right as the investigators are poised to win. If you're about to resign, to spend clues to advance the last Act card, or otherwise achieve the end of the scenario (and even when you're playing blind, it's very obvious when the scenario is about to end), that is the time to Delve. The majority of scenarios have a design that makes it possible for the investigators to hang around for a turn or two while on the cusp of victory.
This has numerous advantages. First and foremost, you don't have to deal with a lot of the encounter cards. Any monsters that show up can be ignored (although you might want to kill them if they're VP monsters), any damage/horror you take that doesn't kill you is irrelevant, etc. Moreover, since you have seen what the encounter cards in the scenario are (if you didn't know them already), you should know if Delving is safe or not. If there's an encounter card that might kill you or otherwise cost you the scenario, you can properly weigh the risks before Delving.
2) If there is a severely wounded investigator who cannot draw the extra encounter card without risking trauma, that investigator can sometimes resign before everyone else Delves. They don't have to draw the encounter card, and they still get the VP!
3) Cards like Ward of Protection, "Let me handle this!", etc., can make Delving at the end of the game considerably safer.
4) The other time to Delve is when you are certain to lose. If everything went wrong and you are inevitably going to have to resign in defeat or take trauma anyway--and we've all been in this situation--there's no reason not to get a VP or two first.
Played in this way, the drawback of Delve Too Deep is not really the encounter cards it draws, which hardly end up mattering at all, but just the fact that it takes up the spot of a card that could otherwise help you win the scenario. But the VP reward is more than enough to justify that, in my opinion. And you can of course replace it by the the last scenario or two.
If you haven't tried a multiplayer group in which every investigator packs two copies of Delve (use proxies to make this happen), I highly recommend it. Your decks will power up incredibly quickly.