One of the biggest complaints has been the slow loading of the Zcoin wallet which currently takes several hours to load and we have been hard at work to resolve this. This was due to additional reverification to prevent double spends because of the added complexity introduced by the Zerocoin implementation.
We are now testing a solution to reduce loading times to a few minutes and have pushed this into the Github dev branch. If successful, the first load upon installation will still take several hours but subsequent loads is aimed to be in minutes.
Loading is also expected to be made even faster once the new MTP PoW algorithm is implemented that is much faster to verify than the current Lyra2 implementation. This is one of the reasons why so much focus has been placed on completing MTP.
You can view the latest pushes to the dev branch of Zcoin here.