For the price, I think the plastic lid is OK and I do use mine, but I'd prefer one of the metal loose fitting lids if I can get one at some time in the future.
The plastic lid is a very tight fit into the cup, but it definitely helps the boiling time, particularly when used with the crusader cooker /hexamine tablets. I can get a good half cup boiled with half a tablet of hexy in about 7 minutes.
I've found that the best method is once the cup has boiled, I take it off the heat and put it on the ground. I then grip the two little handles on the lid, while prising the lid up using a bit of twig in the cutaway section of the lid. If I need to continue heating the cup, the lid can then just be balanced on top of the cup.
It's not ideal, but it can be made to work and I think it's better than no lid at all.
Geoff