I have a 7" santoku with hollow edge, and shortly after I bought it, I kind of wished I hadn't. Not because it isn't a good knife, but because I felt it was redundant. Back then I would have told you that anything I could do with the santoku, I could do with a chef's knife.
That is still true, but now I am happy with my santoku. Not because I wouldn't be able to cook without it, but because of subtle differences. For example, it releases clingy foods much better than my chef's knives. So something as silly as making chicken sandwiches becomes slightly easier. I can slice both the cheese and meat with the santoku, and the cheese lets go of the knife much easier than it does with my chef's knives. Could I use a chef's knife instead? Of course, but it is just a little bit easier, and therefore more fun to use the santoku. I could cite more examples, but I'm sure you get the picture.
However, having said that, this purchase taught me that for me, at least, a knife choice is kind of a personal thing. I should choose my knives based on what I need them for. I bought the santoku several years ago, at a time when they were first appearing on cooking shows in great numbers. I thought "Well, if they all use them, I must need one." Fortunately, it worked out this time, eventually.
But, in the future, no more knives for me, unless I know for a fact they are going to help me do something I do a lot, and make whatever that is easier and quicker.
Of course, this is only my opinion, and only one approach. I recently met a guy who said he had over forty knives, and that he absolutely needed them all. And no, he wasn't a pro, he just worked at a kitchen supply store. I guess he must have gotten a discount. :)