Good good!

But, um, it costs 5. so by the time you can cast it, either your rast swarm of doom has already won, or your opponent really isn't going to care much about an overcosted land destruction that doesn't even really destroy the land if they absolutely need it, or you're pariing it with something that lets you tap and untap the land multiple times and then you've got other evil ways to win already (and hey, if you can tap opponents lands at will, kill them with Psychic Venom for the humiliation value)

So, in closing, needs to be cheaper, could probably safely go on your own lands, and all glory to the horned rat.

Secret Mollusk's evaluation is based on Psychic Venom, but Pooling Venom is a more modern analogue, with a better ability. Personally, I think the idea of putting 2+/2+ rats into play is more powerful than 2 damage. It's incredibly good in a frenemy group game. "Gee, my opponent, it appears you have once again tapped that land of yours and put a rat into play! Guess all my rats are 6/6s now..." Personally, I'm not sure if the card should stay about where it is, or if the partial phrase "as long as it was tapped for mana" should be added, and the cost reduced. Guess that's your call.