Well, they are definitely more in the vein of traditional mystery stories, but I have come to enjoy them quite a bit.
First, if you aren't familiar with the character, Perry Mason isn't a typical mystery novel protagonist. Like Gardner himself, Mason is actually an attorney, though one who professes to love mysteries and, in the words of Hamilton Burger, the Los Angeles County District Attorney Mason frequently comes into conflict with, Mason is a great attorney but an even better detective. Mason is rather mercurial and will turn down big-paying clients he knows are guilty or whose cases don't interest him in favor of working for, or with, people accused of crimes who can't pay, but whose circumstances interest him. Mason, in his own words, detests boredom and paperwork and craves puzzles, making him perfectly suited to the role of mystery novel protagonist.
Very little is shown about Mason himself in the novels and he's more or less used as a spotlight to shine on interesting puzzles Gardner has created. One of the things that new readers of PM novels must be aware of is that the plots of all the novels are, in a sense, all the same. Gardner found a formula that worked very well and stuck with it for more than eighty of these novels. Where the works shine, however, is in the sheer, vast variety of characters, circumstances and twists that Gardner invented and crammed each story chock full of.
These are true whodunnits, too, and clever readers paying attention can puzzle out the solution along with Mason. One of the hallmarks, in fact, of Gardner's PM novels is that there are usually at least two completely plausible solutions: one in which Mason's client is guilty (the theory espoused by the police or district attorney, generally) and one in which they are innocent. Mason, of course, fights for his client and only if he is convinced of their innocence, but there are a few times when I've found myself wondering if the other, equally possible, solution isn't the factual one. It's part of the fun.
As I said, I'm not normally into whodunnits, but I've come to develop an affection for these Perry Mason mysteries. Because the plots are always so similar in many ways, though, these are not novels I can binge on, rather they have become an occasional reading treat and I think that's how to best enjoy each mystery: one at a time, with ample breaks in between for the hard stuff.