I've never tried one myself, but I've heard they have a bit of a learning curve and you need strong kegels to use them properly. I've also heard, however, that they will work with a harness if you need extra stability.
It really seems like they're hard to use period. I don't know of anyone who regularly uses them without a harness (though I'd love to hear from someone who does).
Depends what you are a beginner in. A beginner for strap on sex? Perhaps. For putting things into ones vagina? Definately. You're going to definately need to tone up those kegels, baby. I suggest a kegel exerciser before you use a strapless strapon without a harness. With a harness, it's great for all beginners.