If you are not allowed to talk to the graduate students, then the advisor is likely hiding something, and that is not good. The first year or two of graduate school can be dominated by a very naive perception. Depending on the advisor, he may not have had any great demands of the student. His tasks thus far may have been interesting and challenging but not frustrating and years-long. Beware excessive bitterness - Starting in about the fourth year, students can be prone to an inappropriate amount of bitterness.
Graduate school involves work on a time scale that is somewhat unique.
It is generally not filled with lots of little, immediately gratifying tasks. It is also easy to get caught up with trying to reach a point of resolution that encourages a student to work incessantly for an extended time. This can create a very dark and hostile mindset, especially if you catch the student at the tail end of such a period. It's growing pains, plain and simple. Talk to all of the graduate students - your best information will come from talking to all of the graduate students.