If you intend to visit a lot of museums, the Museumkaart is a very good deal (it also gives access to the most important museums in Amsterdam, such as Rijksmuseum, Van Gogh, Anne Frank House and Stedelijk Museum).
The Museumkaart is valid for a whole year, so if you stay for a longer time, or intend to visit the Netherlands several times within that year, this card is a good deal.
The price is 59 Euros (for an adult, in 2016), half-price for kids until 18, so with 4 or 5 museum visits you can be break-even.
With the Museumkaart, you can visit a museum as often as you like.
The Museumkaart is valid in most museums in Amsterdam, but also in many museums elsewhere in the Netherlands. If you are going to travel the country, the Museumkaart is really worth considering.
If you stay only 1, 2 or 3 days in Amsterdam, the I Amsterdam City Card might be a good alternative. (This card includes public transport in Amsterdam, a canal cruise, several attractions that are not museums and various other discounts).
The Museumkaart can also be used to skip the line in the very busy museums in Amsterdam. You can order online tickets for free with the Museumkaart (you must show the card at entry!).
This is also possible with the Van Gogh Museum, which, strange enough, does not allow the same for I Amsterdam Card holders.
NOTE: As of March 7, 2016, a passport photo is required on a Museumkaart. That is: if you buy a Museumkaart at one of the museums, you get a temporary card which is valid for a month. Within this month, you should upgrade it with your photo, for the Museumkaart to remain valid. If your stay is shorter than a month, and you're not returning to Holland within a year, you don't have to bother.