I just had one yesterday!
It wasn't bad to me, only because I've been MISERABLE with allergies (wheezing, coughing, sneezing, itching, runny nose, etc... you name the symptom, I probably had it!). Last week I started my own countdown with my students (we were practicing counting backwards anyway... anything is a teachable moment!)...
As for how it was, I didn't have the scrape test. I had the back test as the first part... the tech divided my back into 4 sections and injected each section with 6 shots. They weren't big though... just a small quantity of allergen to see if there was a reaction... I HATE HATE HATE other shots (flu shots, hepatitis shots, etc... they make me HURT!), but the allergy shots didn't hurt at all. The only hard part for me was not scratching! I got bored halfway through it and started observing my reaction on my back in the mirror... I took pictures w/ my cell phone too (my DH was working, and he's always interested in that kind of stuff).
Anywho... part 2 of my test was injections of a higher concentration into my arm. The tech said that they inject higher concentrations of what you didn't react to w/ the back test, just to rule everything out.
As far as what they'll test for, I wasn't tested for food allergies, because (beyond bananas) I haven't ever had any reactions to food. However, my allergist talked with me for 15-20 minutes before ordering the skin test...
And my results? (just in case anyone is curious) If it has 4 legs, if it grows out of the ground, if it's a dust mite, or if it's mold, I'm pretty allergic to it. I think 80% of my injections caused a pretty big reaction... (think mosquito bites on steroids... surounded by inflamed skin).
All in all... not painful, just incredibly itchy!
I found lots of comfort in knowing that I'd have a list of things I was allergic to
Best of luck! PM me if there's anything you're curious of!