They can be effected, sure, but if you indicate illness in the checklist of symptoms, Fertility Friend takes it into consideration when it's analyzing your temps. And it's the overall trend you're looking at, so a couple of elevated temps in the middle aren't really going to have a huge effect on things.
My temps were elevated because of illness at like 9-10DPO this past cycle and I did have my hopes up, but in the back of my mind I knew that I was sick and that could have an effect, so I wasn't completely devastated when AF showed. Plus, my temp dropped right before AF showed, just like it always does, despite the illness.
I can't do OPKs, I think I'd get crazy with it. Is that a line? Is it positive? What about this line? Is it darker than the last one? What is that? Need to pee again! Yeah...I can only imagine how bad I'd be. But temping is quick and easy and it helps me pinpoint ov and lets me know when I should expect AF, which is why I do it. Or why I'm still doing it, even though I'm on a TTC break until Feb. I get really anxious about when AF is supposed to come (had a surprise AF in high school that has scarred me for life, I think) and the first few months after getting off the pill, I expected her around CD28 when she was showing up CD25 or 26. Temping is nice because I know exactly when to expect her and there aren't any surprises. I feel much more in control and much calmer about my cycle.
Plus after buying the BBT, I haven't spent very much money on TTCing, since I'm not doing OPKs. I usually don't test until after my expected AF and she's been pretty timely. I get my temp drop right before AF too, so I know she coming. I bought a couple of tests a few months back, but since AF shows right on time, they've lasted a long while...