These days, in order to graduate from a DO school, you need to have passed COMLEX 1, COMLEX 2 and COMLEX-2PE.
You just need to complete COMLEX-3 (1 day exam) to finish.
A lot of DO students take USMLE 1. Some take USMLE 2 CK. I don't see any advantage of taking USMLE 2-CS (since you have to take COMLEX 2 PE anyway) - it's another $1000 and it's pass/fail so it's not like you will use the result to "stand out" amongst applicants.
If you want to take USMLE 3, it's a 2 day exam, and you must have completed the previous USMLE exams (including CS). If you decide to complete the USMLE instead of COMLEX series, not only will it cost you more money (since you have to take USMLE 1, 2CK, 2CS) in addition to COMLEX, but a handfull of states will not recognize your USMLE for licensure purposes (just a handful with california, florida, pennsylvania, etc being among them)
So completing USMLE for licensure purpose makes NO SENSE - either financially or for licensure purpose. I would take USMLE 1 or 2 only if you are interested in an acgme residency and want to show programs that your knowledge is comparable to your MD peers.
If you want your MD colleagues to "respect" you for taking their boards, then do an ACGME residency and take the ABMS recognized board exam since it's considered the gold standard.