1) Think carefully. Intercalating means another year at medical school, with all the costs - financial and otherwise - which that entails. Talk to other students who chose to do one, or not, and ask them their reasoning, and whether they agree with their decision in hindsight. Do your homework about what is actually involved.
2) My opinion would be that if there's a particular subject that you're interested in, want to learn more about, or may need later in your career, then it may well be worth it. Equally if you're interested in the more general aspects of research you can consider it. Don't do one just for the sake of it.
3) Intercal is a year off the treadmill, a year to step back from the inevitability of the medic conveyor belt. It's a great chance for some head space, for re-focussing, and for trying something a bit different. This was invaluable for me.
4) But it is not a year off. For example, my year was composed of a stats course with a final exam, research lectures (e.g. how to run a clinical trial, research ethics) with a final exam, journal club involving presentation and research, writing a research review on a given subject, 3 specialist courses with final exam, and a research project with a final presentation, paper and poster. It's the equivalent of the honours year of a degree. It is a lot of work, and especially towards the end was incredibly busy and stressful.
5) If what you want is a year off, to do some rehab / sort your life out / think about whether medicine's for you, intercal may not be what you need. It's less physical, and it's a change of scene, and you don't have the pressure of ward work, but if a year out is what you need, apply to take one. Most med schools are happy to accommodate this if you have good reason.
6) Intercal will make your brain work. You will encounter more complex ideas that in the normal run of med school. It takes time to remember how to do quality-not-quantity brain work. I really enjoyed this, but I know not everyone enjoys brain ache. It's also very self-motivated (speaking as a PBL student) and that can be very difficult if you are having a bad patch.
7) You will learn a lot. I now feel confident to read and properly interpret papers (a vital skill in an evidence-based service), and I could make a good stab at planning a trial or study. I've also got a good grasp of my specialist area.
8) Myth busting : you will not forget all of your medicine. I was a bit rusty when I came back - I had forgotten some details. But I feel like stepping back for a year allowed my brain to organise and consolidate information. I came back with a better overview of medicine than I'd ever had before. I also grew more confident, and you can notice a big difference in maturity? poise? between those who intercalated and the year below that we have joined.
9) Your access needs may be dramatically different depending on how much clinical work is involved in the early years of your course. Talk to the disability service, and the course organisers early. If you need something to persuade you that access in hospitals is really quite good, do an intercal in a department based over 3 stories, with no lift. See the top 10s tabs for advice on days of lectures and [pending] coping with dissection and lab work for ideas.
10) Before intercal, read Bad Science by Ben Goldacre. Read it again before intercal exams. After intercal run through the main 4 examinations (CV, resp, GI, neuro) and basic principles of history taking. Don't freak out and study all summer!
Anything to add?