Is it better than Amtrak ?
If you have to get just around some of Florida ?
Florida is a huge state with a population that competes with Texas for 3rd place in the nation. The traffic between West Palm Bay and Miami is absolutely crazy.
To answer your question: It should be absolutely better than Amtrak with trains currently hourly and later to be twice an hour in each direction. Currently, you only have Amtrak Northbound once in the morning (two when they return both Silvers to service) and returning in the evening. These times don't even allow for a single trip to Orlando and back in the same day.
Brightline is privately owned and operated. They use pretty new equipment (less than 5 years old), though I would argue not quite as comfortable. I felt like I was sitting on a plastic bench with a layer of leather.
For trips within the major metroplex, Brightline and Tri-Rail (the local government service) are best. Again, Amtrak with one trip in each direction daily can't be relied on. Brightline will be better than Tri-Rail, but much more expensive, but with only limited stops (currently three stations total). Tri-Rail is cheap but very slow and really only useful for shorter trips within the metroplex.
When the extension to Orlando is finished, it will be interesting to see how many people will actually want to go from the WPB-MIA basin to Orlando
Airport. Not sure if Sunrail will connect when Brightline is serving the airport. Eventually when it does, the utility will become more valuable.
When Brightline extends to Tampa, I expect to see a lot of traffic between Tampa and Orlando and Miami and Orlando. It really wouldn't make too much sense to go from Miami to Tampa or vice versa. That can also be done currently on Amtrak. It's about 5 hours and 45 minutes. Brightline may save up to an hour or so. Driving takes about 4 hours. Flying can be done for less than $100.
So, yes. It'll be better than Amtrak, but will it have real value to Floridians? Since they are relying on their funds more than taxpayers money, they are hoping the risk will yield profit.