Portable laptop screen extenders: gimmick or genuinely useful?

The clip-on dual-screen extenders (two 13 or 14 inch panels that fold out from the back of the laptop lid) keep getting recommended to me. I travel a lot and work from hotel desks. Are they worth it, or should I just buy a single portable monitor?

I used a clip-on dual extender for a year. My honest take:

Good: three screens on a hotel desk is a real productivity jump, and they pack into one bag.

Less good:

• They add roughly a kilo, and the whole laptop gets top-heavy. On a plane tray table it is hopeless.
• Most need either two USB-C ports with DisplayPort Alt Mode, or a DisplayLink driver. DisplayLink works, but it costs CPU and can stutter on video; check whether your laptop's USB-C ports actually carry video before buying (many cheaper laptops have only one that does).
• Brightness is usually 250 to 300 nits, fine indoors, useless by a window.
• 13 inch panels at 1080p make text small; you will scale to 125% and lose some of the space you gained.

For travel I switched to a single 15.6 inch portable monitor on a small stand next to the laptop. Lighter, brighter, one cable, and it works on any laptop I am handed. The clip-on style only wins if you truly need three screens and never work on your lap.

One more thing for Mac people: the Apple Silicon laptops with a base chip support one external display over USB-C. The DisplayLink models get around that with a driver, and it is fine for documents, but DRM video (Netflix and the like) will not play on those screens. Worth knowing before you buy the triple-screen version for movie nights.