Congratulations Darkskies. . .after many years you have finally gotten me to create an SDN account. I periodically look at these forums but I’ve never posted before, however after reading your post I felt like you deserved a response from someone who actually took the USMLE during third year rotations. I took my COMLEX at the end of second year like everyone else, and like you I decided later that I wanted to take the USMLE also. I wound up taking Step 1 about two-thirds of the way through my third year, and I was *extremely* satisfied with my score. Let me assure you that it is definitely possible to take step 1 during third year, and it is possible to do well on it. I actually think that taking it after having some rotations under my belt really helped to put a lot of the material in context.
That said, this is a pretty personal decision. I made my choices for reasons that were personal to me, and I'm sure that there are people out there who took step 1 during third year, wound up having things go the other way, and really regret it now as a result. Here are my 2 pieces of advice to you:
1) Don't even think about this right now. The end of second year is so stressful, and when you're under (or recovering from) a lot of stress is the worst time to make decisions and to study. I found it so helpful to just relax for a while and have fun on rotations (yep, they can be really fun) and put the stress of boards behind me. When I began studying for boards again, I had such a clearer head than the first time through. At a minimum, I would tell you to wait at least until your COMLEX score is back AND THE SCORE HAS SUNK IN. The COMLEX might not be the best test out there, but your performance is still an important data point. Don't make any quick decisions either. If you wound up doing well. . .great. Make sure you can repeat that. If not so well. . .think about why before you register for the USMLE. Figure out what you need to change, if that's the case.
2) When the time comes to make the decision, understand YOUR NEEDS and YOURSELF.
2a) YOUR NEEDS: This means that you have to get on the phone with program directors at residency programs you are interested in, in the specialty (or specialties) you want, and ask them how they feel about step 1 vs step 2. We all hear that maybe step 2 can be a sort of substitute for step 1, but the truth is that most of what we hear are rumors or anecdotes about other people who may or may not have been in comparable situations. I chose to take step 1 because programs I want to go to require it, plain and simple. You might want a different specialty through, and even if you don't you might want different programs in the same specialty. The only way to answer this question is to explore YOUR NEEDS. If you're not willing to do that or you don't know what you want, then the important thing to keep in mind is that taking step 1 and doing decently on it keeps all your doors open. Taking step 2 without step 1 will probably close some doors for you, and if you don't do your research there is just no way to know where those doors are. SDN is a good start, but there is no substitute for getting in touch with programs in your field of interest.
2b) YOURSELF. You mentioned that you have been affected by non-academic issues and depression in your post. Are these going to hold you back? The last thing you want to do is take the USMLE (either 1 or 2) out of desperation when you are depressed or not ready and have a bad outcome as a result. Only you can adequately evaluate your state of mind and your level of preparation. I (and other SDNers) can elaborate on our experiences, but they may or not have any bearing on yours. For example, I don't think I had to deal with the same personal issues that you are confronting when I took my test. That said, I think anyone who doesn't admit to at least a little situational depression from time to time in med school is lying to you. Talking about your issues on SDN is a good start, but you have to seek out the right resources and get the support that you need to get through these issues. That really should be a prerequisite to this decision. If your confidence and state of mind are in a good place then you have a much higher chance of success at any level of preparation. Maybe this touchy-feely stuff isn't what you were looking for but it does make a difference.
I think you should to focus on giving yourself some time and sorting through these issues before you move on to thinking about practical issues like test timing, studying strategies, etc. I know from my experience that taking USMLE during third year can be done successfully, but I can tell you that before you confront the challenges that everyone confronts in taking that exam, you have to decide if you need it and if you think you can do it.
I hope that helps. Good luck to you!