Pay for your Training
Pay for training (PFT) is a bad thing.  I am specifically referring to operations in which you pay to go through initial aircraft training and fly right seat.  Yes, you not only pay for 121 initial (BAD!), you also pay to fly from the right seat.  The bottom line is you are occupying a seat that the company should be paying you to be in!

Part 121 pilots are in a constant struggle over wages.  Companies paint a picture that the only reason they take quarterly losses is that they pay the flight crews too much.  Negotiating capital has to be spent to combat this.  These pay for job (PFJ) operations hurt our negotiating positions on wages, as do the pilots that come out of these programs who some might argue are more likely to support concessions (after all, they'll fly for free!).

It is hard to combat these kind of programs because flying is more than a job for a lot of people. It is a dream and a passion and some of these people are more than happy to pay for the "privilege" of flying a beech 1900 from the right seat. 

You asked for Pros and Cons, so:

Pros


You learn an aircraft and log some SIC and TT, maybe even turbine.


Cons


You are helping depress wages across the industry.
You are enabling the company to avoid paying a wage to half of its pilots.
You will probably carry a stigma when you move up in the 121 world, at least at the regional level (FO's I flew with were very reluctant to name their former operation if it happened to be [REDACTED])


Don't go the PFJ route.  Find something to do that will pay you a wage to fly and build hours to get into a regional 121 seat.  Once you have your commercial (and cfi) certificates, you shouldn't be paying to fly any more.  You'll also find once you get into the 121 world that paying to accelerate your way there  perhaps wasn't worth it.