DEET is effective. What I don't like about DEET has nothing to do with any health effects it might or might not have. The problem with DEET is two-fold:

1. It's most effective at high concentrations, or with a controlled release formula. (The CR stuff is harder to get.) Unfortunately, especially with high concentrations, this stuff WILL interact (i.e. melt) some plastics. Maybe Thermacell is harder on a mirror than smoking 10 packs of cigarettes per night in your obs, this remains to be seen. However, I am quite certain that DEET will screw up some stuff made of plastic, because it's happened to me. The controlled release stuff is MUCH better about this - you can get it from REI or other camping stores. (3M makes it - it allows you to use much a lower concentration of DEET.)

2. Other people in the dome. I may not be afraid of DEET, but as soon as someone notices "hey, I picked up this plastic thingie, and it kind of melted, this stuff has to be really bad for you!!" then it's all over. It doesn't seem to matter that I point out "well, yes, it was bad for that bit of plastic, but YOU aren't made of plastic, are you?"

It is substantially cheaper to use DEET than a thermacell, unless you just have a massive number of visitors.