I'm a DO who took the USMLE and did well on it. In spite of that, I didn't match this past year.
Still, I don't regret taking the USMLE. I received interview invites at pretty much every place I wanted, and I think that was mostly because of my USMLE scores. I think the biggest reason I didn't match was that I made some strategic errors in deciding which interviews to accept and which to turn down. I underestimated how competitive applications would be this year and took it for granted that I would match within my top 3 choices like people usually do. I wound up wasting some precious interviewing time on a couple of programs that turned out to be probably not very open to DOs, and I was somewhat limited geographically by wanting to stay within a few hundred miles to an ill relative. If I had known then what I know now, I would have chosen my interviews more carefully, made my rank list a little longer, and I think I could have matched.
Since everyone had told me that psych was an easy match and a "buyer's market" (and even a PD told me that it was "up to me" whether I matched with them or not), I never considered that I might go unmatched and was blindsided by it when it happened...so the lesson is, hope for the best, but be prepared for the worst (i.e., know what to do if you have to scramble).
All that being said, I still recommend taking the USMLE. First, because you are perfectly capable of doing as well as a MD grad on it if you study the way MD students do for it. There is no big secret about what MD learn in their first two years that we DOs aren't privy to.
Second, because I think it does help at some allopathic programs to give them a way of easily comparing you to the MDs. One of the allopathic places I interviewed at had a DO on staff, so you would think it would be DO friendly, but that was also the only place where an interviewer outright asked me what my COMLEX score meant because that particular interviewer had no familiarity with the test at all. If I had not also had the USMLE to show that person, I would have gone on to wonder if their lack of familiarity with COMLEX was why I didn't match there. It's good if you can look back and know you did everything you could to optimize your chances, regardless of the outcome.
On the issue of DO discrimination, I think it is hard to generalize because you're ultimately dealing with individual people's preferences. Even if a program has historically been DO friendly (or not) that can change fast if there is a change in leadership at the program. At one of my interviews, one of the interviewers acted very impressed with my credentials and we seemed to hit it off unusually well, so I thought I was a shoo in there...but then after my interview one of the residents sent a mass email out to applicants talking about how the chairman of the department had initially intended to get away from recruiting DOs and FMGs to raise the prestige of the department (oh, but don't worry, because they had talked him out of that plan and the resident reassured us that DOs were welcome after all). I ultimately didn't match there. I can't say for sure if that's because I'm a DO, or if it's because they just had so many good applicants that I just got the short straw by random chance, or if I was oblivious to totally bombing every other interview but the one that went so well...but I had no idea until after I had traveled there that the culture at the institution was
potentially becoming DO-unfriendly.
For that reason, I recommend not blowing off the DO residencies. Even if they aren't in your dream location or whatever, they'll still get you to the ultimate goal of becoming a psychiatrist, and if you don't match in the MD match you'll regret not having given them a chance.
If by some chance you don't match even with all the best planning...well, not matching isn't the end of the world. Even though I was unable to scramble into a psych spot, I found an osteopathic internship at a program that is very supportive of my psych goal and I expect to be better prepared for the match next year.
All the best to you.