Darkcoin currently makes no sense.
the 2 main things that set it apart from other coins are not yet implemented:
Encrypted transaction network: Work In Progress
Anonymous blockchain using coinjoin technology: Pre-Alpha
Zerocoin will be released soon and it has all of these things taken care of .
and apparently can make any coin transaction anonymous through an integrated layer
I think we need variety in the cryptocoin space. Two anonymous currencies are better than one and much better than zero.
DarkSend is in a working state on testnet, so we're making good progress. I wouldn't call that non-existant. Also, Zerocoin uses exotic mathematics that could have some unforeseen hole in them and DarkSend uses a completely different implementation that is pretty straight forward and relies on tried and true mathematics. So again, 2 is better than 1.
Checkout this link to see the mathematics I'm talking about in Zerocoin:
https://github.com/Zerocoin/libzerocoin/blob/master/AccumulatorProofOfKnowledge.cppZerocoin's proof of work is in the 2 kilobyte range if I remember correctly, per transaction. That's alot better than it was for their first proof, but it will still cause alot of bloat in the blockchain. Darksend's method of anonymity on the other hand doesn't have very much bloat at all.
Don't get me wrong, Zerocoin is very cool, but obviously I think Darkcoin has it's place.