I've used MyStocks Portfolio exclusively since WP7 but I have two complaints.
1. It looks blah. It serves its purpose great, but it could do alot more to make it look appealing and really use the WP Metro styling.
2. My biggest complaint is that it doesn't save your portfolio and keep it linked to an account. So if you get a new phone, reset your phone or uninstall the app, you need to manually enter your holdings all over again.
I had to reset my phone last week and I haven't gone back in yet to manually add them again. I am going to test others to see if there is a better solution to my problems