They are great. Results in one day
I agree with Howard. Very convenient to use but tends to be pretty expensive relative to other options and their follow-up support is not that great. I have used the services multiple times. Last time, test on Wednesday, results back late Monday, called on Tuesday to speak to the doctor and they didn't have an appointment until late Wednesday night or early Thursday morning. Not exactly the response time you want when you have questions. If it is privacy you are looking for, probably better to go to one of the immediate care centers and handle it via self pay.
If you know what test you want it's pretty slick. My one recommendation would be to avoid taking a test a day before the weekend it can delay your results by a couple days and if you are having any anxiety it's hell.
Forgot to mention another downside of online test brokers: usually they provide for gonorrhea/chlamydia testing only by urine, or maybe by urthral of vaginal swab. Those tests don't detect infection at other anatomic sites (rectum, throat). If those sites were exposed, urine/urethral testing is pointless.
Most if not all online STD testing services work pretty much the same way. They don't have their own labs and don't actually do any testing. They have contracts with commercial labs, most frequently the two main national labs, Quest or Labcorp. The tests offered therefore are usually accurate and state of the art.
Where online testing often falls down is in implying a need for more tests than are actually necessary; and in the availability or quality of follow-up contact to help interpret test results. Most people can get less expensive STD evaluation in a doctor's office, especially if you have access to a public health STD clinic. The cost of the visit plus the tests is often less than what the online services charge for testing alone; and with a professional visit you also get better guidance on which tests you do and don't need. As well as the physical exam that often picks up issues the tests are not designed to do.
In any case, I'm sure the website(s) you're looking at will describe the specific tests they offer. If you go ahead with HSV testing, make sure it includes HSV1 as well as HSV2 IgG test; and do NOT have IgM testing, which is inherently unlreliable and often misleading.