Very easy. We first bought Sunpass for convenience when staying in St Pete Beach not necessity since there's a Toll Plaza there. We bought the temporary transponder (plastic one with suction cup not sticker) at CVS or Walgreen and 24h after we set up our account online it was active. In July we saw a sign at the I-95 Florida Welcome Centre that advertise they were selling the transponder there and they were active right away. Might be worth checking it out.
Sunpass is not mandatory but it saved us hours of driving, especially between Ft Lauderdale where my parents own a condo and Orlando. FL turnpike is much faster to go to Orlando (or Ft Lauderdale/Miami from Orlando) compared to I-95/I4 and a Sunpass is mandatory to take it. My uncle went on it without a Sunpass and paid a very expensive fine. I also have mine on auto-renew so when my balance gets down to 10$ it adds an additional 10$ automatically so I know I always have enough money to pay the tolls without carrying tons of change. I also like that you can have multiple license plates on your Sunpass account. You can easily access your account online or can use their app on your smartphone (I have an iPhone). Finally, Sunpass is accepted at all tolls in Florida, including E-Pass tolls that are different than EZ-Pass that you will see everywhere else in the USA. There's many tolls around Orlando and we often use them to avoid I-4 as much as possible. We HATE I-4 and have seen so many accidents on it that we try to avoid it. Too many tourists that don't know where they are going mixed with crazy Floridian drivers.