I tested darksend today!
I must say, I like it that you can premix your coins and then use that "dark balance" for sending anonymous transactions

Pretty impressed by the GUI as well.
My only remark (also related to the thing below) is that it took nearly 3 hours to mix 2 DASH... That seems a bit slow, but hopefully if we get more masternodes online, that won't be an issue anymore. But I'm willing to wait a bit. Privacy has a price, in this case it's waiting time.
Someone correct me if I'm wrong, but isn't mixing slow sometimes because of a lack of people mixing, not the number of masternodes?
Yes, it is wallet funds that mix. Masternodes coordinate. We have several liquidity providers(wallets with funds available to mix) online to help with mixing. So there are funds to mix with. Please make sure you are running the latest version. Also it takes along time for the last few % to get mixed. So mix with more that what you need and stop it when you have enough. It costs the same(0 or .01) no matter how much you mix.
The biggest delay with mixing seems to be unconfirmed transactions because all the mixing uses no fee transactions. It can look like your mix is stuck for 15 or 30 minutes waiting for this transaction to confirm. The suggestion I have is to mix when the network load is low.
Another thing I want to mention: when testing a transaction, it costed me 0.1 DASH as a fee. I know that with transactions, fees are involved, but 0.1 DASH (around 0.001 BTC or 0.25 USD) is pretty hefty. Will fees be lowered someday?
Any funds you send out with a darksend funds will be denominated with 0.1, 3.2, 10.3 etc.. So a .001 transaction rounds up to .1 and you are charged a .099 fee. If you send a .1 amount or just under you won't get the .1 fee.