I'm not a huge fan of cafeteria-style dining and the Ikea Cafe seems to be less than stellar even within that category. That said, the prices aren't bad--especially if you order the daily special.
They offer a small selection of entrees with little room for modification. I tried the vegetarian meatballs and was particularly unimpressed. The food they offer is no better than what I can make at home (and I'm by no means a good cook!) so would rather make my own (I think mine would taste better) and save the money. All their food tastes quite bland.
The cafe is great for employees and those who spend hours in the store so need to catch a quick meal. For the visitors (and I do recommend taking a family field trip to Ikea!), I'd skip the meal and go straight for the $1 ice cream cones in the basement.