I've recently made the switch back to using Google Tasks instead of Wunderlist for my tasks manager. There are pros and cons to both, but the main reason I switched was because of the syncing issues between mobile and desktop I was having with Wunderlist.
Now I think Wunderlist is a great app, it looks amazing, has some nice functionality and the mobile (I'm using Android) version has really improved over the last couple of releases. The problem I was having was that I use the mobile version to set up tasks in the morning of what I want to achieve or when a idea pops into my mind, I'd record a task in Wunderlist. Then later in the day I'll be on my laptop and I'd want to access the tasks I set earlier in the day. I was have problems with the desktop version not syncing with what I had set in the mobile version. When I tried syncing it failed, I would eventually have to re-log into the app and that would sync my tasks.
So I started looking for alternatives, what I wanted from a task management app was, good syncing between desktop and mobile, a nice looking, feature rich mobile app and a desktop version. Google tasks has good syncing, some nice features like reminders, links in well with my calendar. For the mobile app I'm using GTasks which looks great has good features like reminders, alerts, but best of all syncs well with Google tasks.
The only slight downside on Google Tasks is a desktop version, Wunderlist has a great looking desktop version, there isn't something like that for Google tasks. Ones I have seen seem to wrap the browser version of Google tasks, but haven't created a specific UI for the Google tasks using the API Google released, which is a shame.
On the whole, I am happy with the mobile version and how well it is syncing with tasks in Gmail and I'll continue using it for a while, hopefully a better desktop app will come out.