InterDigital is a very unique organization. Their professionals are very sharp, and the company occupies a unique place in the wireless sector. They also appear to have a genuine caring for their people and express to make long term committments (e.g. actual life) to their professionals. While this can be good it's fascinating that they've achieved the pole position that they have given the amount of time and follow up it took for their interview process (e.g.a very long time).
To be fair, while taking a long time does make sense given the commitment they propose to make it's difficult, even at an advanced stage of your career, to present the kind of stability that they'd like to see (e.g life some place else). As an example you're obviously looking to make a move by interviewing so if you're there no matter what reason you have for leaving you can't come in and say I'm never looking to leave where I am which is what they'd want to see.
In short sharp people, professional and very nice people that care about where they work and keeping it a nice place while it also appears to be very stable, and a good recruiting process where you're exposed to a lot of folks, but they need to move things faster from start to finish to get to a decision and get back to you.
I'm not sure this last part will happen as I just asked two days ago but full disclose about why they don't extend an offer would also make it for a first rate operation. Provided the chance I'd definitely go back and work there.