I guess I'll weigh in here, well, just for the sake of weighing in. I agree with luckydave and Brian on this one. People have different environments to deal with, different crew (with different abilities and different trainability), and different personal levels of hand-eye coordination. Spending money on something that makes boating pleasurable given those various factors is money very well spent. For many of us, this isn't a rodeo contest, or challenge to our manhood, it is Pleasure Boating, in that order.
My boat came with a thruster, and though we took the time to make sure it was rated correctly for the boat and worked within specs (continuous duty cycle), I personally didn't assign any particular extra value to it. With the exception of one Grand Banks we chartered, all the twins I'd ever used didn't have them.
When we had to move the boat from an odd slip over to a slightly tricky- to-approach lift as part of the survey, my broker, an extremely experienced captain and Hatteras handler, was elected to do the job, being something of a legend.
He hammered that thruster like an eager game show participant hitting the buzzer. After I got the boat, I had another very experienced pro captain, who had done a lot of the work on the boat as well as delivered it a few times, take a half a day and show me best practices for handling it. He too used the thruster with alacrity. They both gave me the same answer when I made comment/question to the effect of "gee I thought you pros didn't need to use those 'cheaters'": "Hell yeah, I'm gonna use it, if it's there and gets what ya wanna get done better, USE the SOB! They're great!"
One of the guys went on to say "I can do everything I did today on one engine too, but it'd pretty dumb not to use both of them, wouldn't it?" Of course, we went on to do some one engine exercises too, which I had some forced experience with earlier in my career.
I have to admit, I don't use the thruster very much, though often times I should, and I always have it on and ready when near the docks. It has come in extremely handy in a few situations. I am an average boat handler at best; I have mild dyslexia, so am one of those like Pascal mentioned that adding the extra element sometimes can confuse things, so I forget to use it.
So I guess my advice is get some experience using one if you can, and if it makes life a whole lot more fun for you, go for it. You'll have plenty of company amongst the grizzled pros as well as us pleasure wussies.
Oh, and what Pete said on size. Go "up one" on thrust power if you can.