I bought a Holga last year, and while I don't use it now, it certainly was fun to experiment with and I'm glad I still have it. Several posters here have said how unpredictable Holgas are, but that wasn't my experience. My Holga focused correctly and the exposure was predictable too. Within a few rolls (assuming you're developing your own film) you'll get the knack of how to develop the film to get a decent exposure.
Some posters here are acting like there is no relationship at all between what the photographer does and what the Holga delivers. I don't think the pros that are using Holgas are surprised at the results they're getting at all; they're not slot machines, they're cameras, and within limits, can be controlled. They just have a unique signature. So to the OP who asked if he should just use his existing MF camera, I say go ahead and use it if you like it. If you want to try a Holga nobody's holding you back, and they're cheap.
A holga is just another brush in the paintbox. I don't consider it a toy unless that's how I'm using it; it's just a simplified and very inexpensive camera. The fact that young photographers use them is a bonus and I'm glad they're interested in film, and photography.
Sometimes I let my 9 year old use my Holga. He's done quite well with it! 🙂