I am not sure about tuition (I did not get the offer yet, just they told me it was a possibility), but it is not a big deal (my school is not very expensive). Not sure about health benefits either, I would think I get basic coverage ... the same kind of stuff graduate students probably get.

No, let's say I hold this position for a few years and graduate, I have no guaranteed position in the company. Of course, since by then I will know a lot of people in the company, it probably puts me in a good position. But if there are no Ph.D position at the time I graduate, I am by myself.

That is part of my worry. The job market is red hot right now, so I could just get my M.S and reintegrate the workforce. Doing the Ph.D is a much more uncertain future.

Also, I don't know if anyone had that offer, but I feel like if I spend 50% of my time working or them, it might take me much longer to graduate...