"Street Captain" implies exactly what it seems. You are hired "off the street" as a captain. It's usually done because not enough FOs are qualified to upgrade. This is popular at places like Gulfstream where FOs pay for their seat, then leave for another airline. It's also popular at airline who hire extremely low time FOs who will take 3-4 years to get the time to upgrade, yet face huge captain attrition. This is most of the regionals now. Expect to see street captain hiring increase in the future.

I would opine that unless you're already typed in the airplane in question, with part 121 experience, it's not a good idea. Your training will be extremely difficult, and you will find yourself held to a higher standard, especially by the FAA. The typical washout rate for street captain programs is above 70%!

Then, once you make it to the line, you'll be a very new pilot flying with FOs who are of very little help. And you'll continuously backslide in seniority as FOs above you upgrade. Expect to spend a VERY LONG time on reserve or flying crappy schedules because of your lack of seniority.

So to sum it up, don't do it.