I've been here a few times, and I'm a fan.
I am impressed with the amount of money they spend filtering their water to be chemically similar to New York City water. If you know some of the science of baking, the minerals present in the water will affect the gluten development (among other things) and this can have a noticeable effect on the resulting pizza crust.
Any place that is fanatic about their pizza as Grimaldi's clearly cares about the product they produce, and Grimaldi's has a great product.
The service is so-so. One time I went the guy kept trying to be funny as he tried to up-sell us on apps and desserts. But he did it in such a way that it was more irritating than funny, and you could tell he was just going through the motions. I thought it was sad to see an employee working who was clearly not enthusiastic about his job. (We all have off days, so I'm not going to slam them on that one interaction.)
I went on Monday and I told my waitress I wanted a white pizza with ricotta cheese and sun dried tomatoes. When the pizza arrives: ricotta and red bell peppers... at least she got the right color! ;)
I've also been when the waitstaff was just inattentive and didn't fill my drink when it needed to be filled. (If you've read my other reviews, you know that's a big deal for me when it comes to sit-down restaurants... if you aren't going to have a dispenser out for your customers to fill themselves, then your waitstaff needs to be attentive.)
Overall, it's great. I keep going back. The service is not what I'd consider deserving of a 5-star rating, but it is arguably better than most. I just don't want to be frivolous with my 5-star ratings.