Oh man, I haven’t experimented with those myself, but I’m all about trying out new things! I feel like you really have to just give these things a shot to really find out if it’s something worthwhile or not, as too many people have skewed opinions and situations. Worse case you can always just stop and/or sell your car, right? So I say give it a shot for a month and then report back your findings ;)
I created an “Every Day I’m Hustling” shirt and sold enough for the campaign to ship, but didn’t earn much on the experiment because of some not-very-effective Facebook advertising. Still, some opportunity here and a friend of mine is doing REALLY well with this. If you can create awesome designs and reach the right people with good Facebook targeting, Teespring can be very profitable.