Well OP, here's the skinny. I never worked fulltime for IBM but I have contracted with them over 1/2 dozen times over the decades.
You have to watch yourself with IBM (all clients, really) or you will be taken advantage.
Case in point:
Make sure your consulting contract has an overtime pay rate clause - as in 1 1/2 times your hourly rate for over 8 hours/day; 40 hours/week. IT'S THE LAW. Do NOT fall for the "professional day" c-ap.
That's just an excuse to get excess labor hours from you (where you are 'suddenly' treated as a "fulltime" employee) and IBM not paying for it. You're a PAID HOURLY contractor - so act like one.
If you get s—ered into the "Paid time off/ "compensating time off" scam, you'll be accumulating that as more "mission critical" projects keep popping up and you will have to beg and grovel to use iyour accumulated PTO. Management has a dirty habit of "suspending" PTO/comp-time for projects or for support coverage - been there; seen that. And if your contract ends, you will LOSE that "PTO" . So - Avoid it and get an understanding about payment for excess hours past 8 per day and 40 per week as well as "on call support" and weekend work before you start. Don't ever say "No" to it; but make it clear you intend to be paid. IBM will probably not go for it - in which case, you get to enjoy your weekends as well as have a good night's sleep each night.
If there is on-call and weekend work - that's overtime (BY LAW) and there is actually a schedule (made by IBM but not advertised) for contractor payment in those circumstances.
One annoying thing you may encounter depending on the management boob overseeing you:
IBM often has "feel good" events like afternoon lunches, picnics, house events, etc. Sometimes these management jerks think they are doing the project a service by saying you are not invited - "it's for "fulltime" employees only". I've experienced my immediate IBM management shutting me out; letting me attend on the sly, as well as inviting me outright. I never let any one of those decisions bother me. Let them have their goddamn event. Just give me the money.
Forewarned is fore armed. Stay focused.