Intense! Intense! Intense! I did the summer program in 2012, and it was truly a meat grinder experience, although my Mandarin Chinese skills did improve. You sign a language pledge and agree to speak only Mandarin on the premises--no kidding! That includes the break room, the restroom, etc. Classes are small and to the point: 1 hour in length, with 2 classes of 4 students each + a 1-on-1 session. You are required to prepare before class--including being familiar with the vocabulary and the story--and leave the textbook closed during the session, so unlike the big university lecture hall experience you may be accustomed to, you really can't hide behind your book in class! Expect 4-6 hours of homework a night. As a friend who had completed the standard Academic Year program (2 semesters, 4 hours daily) said: "You need to only be doing ICLP and nothing else the whole time."
You will be tested at the beginning of the program and then placed in a class at your level. I believe I got myself placed a notch or two higher than my actual ability, since I'd been studying 1-on-1 elsewhere and had seen some material that was probably more advanced than my actual level. Also, I'm a university prof in Taiwan and have taught test preparation for a living, so I believe I managed to get myself into a slightly higher level than I should have been in. I believe I may have nailed a few more test items than I should have, plus my pronunciation is fairly good, so I may have somewhat fooled the interviewers on the oral part of the process.
Class was tougher than I expected, but I decided to tough it out--big mistake! I ended up finding a private sector school to help me with 1-on-1 homework assistance to get me through the summer. The experience was bit like swimming upstream in a waterfall made of quicksand! In retrospect, I should have taken the opportunity that ICLP offers to rethink and renegotiate your placement early in the semester. I did survive, though, and my Mandarin is better for it. Also, ICLP alumni have the opportunity to buy textbooks for self-study after they leave. I have studied my course materials at the same private sector school that helped me survive ICLP, and I still study with them, so at the end it all worked out OK.
I'm telling this story to encourage you if you're thinking about ICLP. It's a wonderful school with first-rate faculty. If your circumstances--time available, cash on hand, etc.--allow, this is an opportunity to make great progress with your Mandarin Chinese. I studied 2 textbooks, 1 in each group class, with the main textbook being taught once more in the 1-on-1 session. You'll be pushed beyond what you can absorb, but at the end, it all works out. To this day, when I hear or use one of the 50+ weekly vocabulary items I learned that summer, I say to myself: "Wow! That was a $3700 word! [the cost of my summer program in 2012]"
ICLP generally does only group teaching, because the physical facility is small and they have only so many teachers to go around. 1-on-1 is limited to the 1-hour session that's part of the program and also for some very advanced students and alumni with special needs for work, academic research, etc. You will have a student ID from National Taiwan University, which can get you student rates on public transportation, student tickets at the movies (if you have time for movies!), and access to other NTU amenities and facilities (amazing gym!).
All in all, it's well worth it to attend ICLP if your circumstances allow. You'll meet interesting folks from all over the world who are learning Mandarin for a variety of reasons and your Mandarin Chinese is sure to grow. Go ahead--swim upstream in a waterfall made of quicksand! It will work out OK in the end.