To be perfectly blunt, many of JDM's standalone novels are not novels in any true sense. Frequently, they are novelettes in the 10-15k word range prefaced with 35 to 40k words of (often unnecessary) backstory, simply to get the book to the required word-count range.
Travis McGee novels are not like that at all. They are true novels with plots and subplots (sometimes multiple subplots) and while the solution to the mysteries occasionally comes out of left field (such as The Quick Red Fox), JDM does seed these books with clues that an insightful reader can build upon towards figuring out the solution.
That said, the series stands apart from many other series by JDM's contemporaries in that they have continuity between them and Travis McGee, the unlicensed detective who calls himself a "salvage consultant" (because he will return your lost valuables to you, whether they are physical or not, when you have no legal recourse for half their value), changes along with them.
In the first novel, The Deep Blue Good-by, McGee is a professional beach bum without a care in the world. By the end of the novel, he's been emotionally devastated (leading to the events of the second novel), and it takes a couple more books for him to recover sufficiently - only for JDM to throw him into the wringer once more. The books deal with a variety of situations and crimes, but the themes of loneliness, dealing with encroaching age, and carving a place for yourself in a world that doesn't need any of us--at least not individually--resonate throughout the series. And with each book, McGee gets a little older, a little wiser, a little sadder, but he also becomes nobler.
He's been described as a modern knight-errant, and that's not a bad comparison. He is a roughneck, and perfectly capable of cracking heads, but it's a last resort, and he fells bad about it. He respects people. He especially cherishes women. Unlike the heroes of many detective series, he doesn't simply fall into bed. He has to be in love "at least a little bit," as he says in one novel, and while his relationships don't work out in the long-term, he remains friends with a surprising number of these woman and often they make repeat or even recurring appearances in later books as supporting characters.
McGee is probably one of the most realistic detectives in any of the works I've read - and I've read literally thousands of P.I. stories and books. That aside, the books are JDM at his absolute best. I've read more than half of the series and never found one yet that isn't at least good, and several of them are truly excellent.
I think what I'm trying to say here is that I love this series and if you like mystery fiction with something more, you will, too.