My wife and I were recently at the Van Gogh Museum and went to the front of the line with a Museumkaart. The museum was totally sold out except for people with a pass card. I don't know if the I Amsterdam Card lets you jump the line. If it does, there's no reason to make reservations.
I wasn't wowed by the Van Gogh Museum. I liked the Kroller-Muller Museum's Van Gogh collection and presentation more. It was also not very crowded. The Amsterdam VG Museum was packed.
As with all passes, you have to do the math. The I Amsterdam Card doesn't cover the RIjksmuseum or Anne Frank House. The Museumkaart does but it doesn't cover any transportation. We used the buses, trams and metro a lot. We had a TripKey Pass and went all over The Netherlands over a little over 2 weeks. Plan your sightseeing and realize you can save valuable time with a tram for a few euros a day.