I really don't see Brightline to carry many people from Miami to Tampa. It'll mostly be MIA-MCO, and MCO-TPA or points between. It just won't make sense to take a 5-6 hours train journey when you can drive that in less than 4 1/2 hours.
It really depends on what your mobility story is at the destination, and why you're going.
My original Brightline to MCO trip at the beginning of September 2023 (which ended up on AA because of the opening delay being pushed back twice) would've been either a $2.25 or a $20 ride to downtown Miami, a $158 round-trip to Orlando, and a $40 ride to the Bonnet Creek area for a conference. Driving would've been 234mi, or about $25.74 in my 2012 Mustang V6, plus $20/day parking; cheaper for the four nights we were there, especially if my friend paid for gas, or if we wanted to go other places in the Orlando area sure. (Aside: we did rent a car for a day to try and fail to go to
Taste of Chengdu, and did go to the excellent Chuan Lu Garden instead.)
I did finally take it at the beginning of November, booking first class, working during the ride, and renting a car at MCO, for a two-day work meeting in Melbourne. I sat down and did the math on this today, and for my situation (working hourly, renting an EV, and charging it at my parents' in exchange for buying them dinner I'd buy them anyways), Brightline coach is cheaper for a four-day trip, and first class is still cheaper on a two-day trip.
I did enjoy some intangible benefits, like supporting rail travel, minimizing exposure to South Florida traffic, a multiple-day test drive of a car that's not eleven years old, and lots of whiskey on the ride back south too.
For grins, I priced out flying AA too, and it's worse any way you slice it, because the flight isn't long enough for me to get anything done, and basic economy a month out isn't significantly cheaper than Brightline first.
The costs do get worse as soon as you have multiple people and aren't billing hours, but that's where cars start to shine traditionally anyways. On the flipmode, I'm planning a trip in a couple weeks driving north with someone, she returns to go back to work, and I stay another couple days (boat parade!) and take the train back.