Ive used them on VHF and UHF without many issues, but HF is a little different. I liken it to taping a dummy load to the inside of the window. Will it radiate? Yeah, kinda, but so will a 6" piece of damp string.
I had one on my wife's van for a few years, and it was fine, since she pretty much only used it when we'd convoy - so car to car, line of sight, less than a mile. If that's all you're looking to do, and space is a big concern, it might work for you. Just depends on the application. It certainly isn't going to give you range anywhere near comparable to an actual antenna.