Having read the rather mixed reviews on here I thought I'd bite the bullet and hope for the best. Whilst I'm not bitterly disappointed, I wouldn't recommend this product to others. Firstly, I agree with other reviewers that the cage for the tea can often fail to properly hold in some of the smaller leaves, which means an unpleasant residue at the bottom of the cup. Not the worst thing in the world, but not exactly what you'd expect and not a problem I've had using the more common separate strainers. In addition, having purchased the cup only a few months ago, it has sprung a leak and there is now tea trapped between the layers of the cup. Compared to other vacuum flasks I've owned this one was always a bit on the rubbish side, and it seems that the overall build quality might be at fault.
I am now going to throw this out and buy a separate strainer to use with my other vacuum flask. If straining loose leaf tea on the go is an absolute must, then this product might still be worth it. For me, however, I think you're better off just taking a few minutes to use a separate strainer and in emergencies just using a regular tea bag.