if you do, make sure (almost essential to score highly) to get the usmle worldbank question - this is expensive and time limited, so plan wisely
if you're going to do it, do it right - you only get one shot (unless you tactical fail, but that's silly) and the score is the single most important discriminator by far, for internships (it was already competitive for IMG jobs - they give preferential allocation to americans, then you compete with the rest of the world; now there's even less jobs due to recession)
depending on how busy your days are, it's said you'll need 3-6m
if you're at imperial, i'd recommend leaving it until after path lectures - if you're elsewhere, perhaps do it at the start of final year, but seek out graduates from your school that did it and understand your curriculum better; at imperial, we don't get taught histopath, extensive data interpretation/microbio (which is heavily featured), or any specialties which also feature, until 5th year (path)
if you're thinking about it, why not do it? it's a good challenge, and the depth/breadth of preclinical knowledge it requires can only do good (it'll help on wards, in research and *apparently* in MRCP) - plus it's always nice to have that option open (I believe it's valid for around 5/10y); if not, it demonstrates considerable volition and time management
essay over. good luck! start with registering on ecfmg, buy the first aid usmle step 1 book (not cases, maybe Q&A also; pm me and i'll direct you to a place you might look, should you so happen to, where you may find a pdf copy - but the 2012 is now in colour and a lovely book to have imo)
edit. spelling
seems a popular post i'll add some other tidbits - once you register with ecfmg, you need your school to communicate with ecfmg; this whole process takes 1-2 months - finally you are deemed eligible. you may then choose a 3 month slot - then it's real. from that point, you may choose to sit the exam at any date, but within that slot.
it is an 8 hour exam, with 1 hour break, performed in a computer center (UKCAT style) in the UK. results take 1 month. the timeline is important, as some snazzy electives require a score.
step 2 cannot be performed in the UK, and requires travel to america, dubai, some other places; unlike the step1/preclinical, step 2 is clinical knowledge and skills (think OSCE and EMQ papers; apparently, much easier than step 1) - from here onwards i'm unsure of the details. in total, licensing will cost you $5-6000