I've stayed at the Thunderbird for a week during March twice now with my family, once in a poolside room and once in a Gulf view suite. We have young kids and appreciate how easy and relaxing these vacations were. The pools and hot tub are a big hit, and the beach is beautiful and so big that it never seems crowded. We all like the nightly live music that the Thunderbird offers at the poolside Tiki hut around sunset.
The rooms are not fancy and the place is certainly dated, but it is a great location with good restaurants and grocery and drug stores within a couple of blocks. It also backs up to a nice mile or so long paved trail along the beach that is good for stroller walks. The in-room kitchenettes are a big plus so that every meal doesn't have to be a restaurant meal.
When we stayed in the Gulf front suite, we were put next door in what used to be a different motel (bought out by the Thunderbird since our first visit). I was disappointed by that because I thought we were getting a suite in the original Thunderbird with a balcony. So beware of that--the Gulf front rooms that are original to the motel have balconies and the ones in what used to be the motel next door do not.
One other negative for us was the no one under 18 in the hot tub rule. On the cooler days in Spring when it is too cold for the beach and pool, it would be nice for the whole family to be able to enjoy this amenity. The water was pleasantly warm, but certainly not too hot. This negative for us could be a positive for those who are on an adults only vacation. Perhaps the management could institute some family hours and some adults only time.