I picked up one of those cheap Bluetooth fish finders for a pond near my place just to mess around with it, and honestly they're more functional than I expected for casual fishing. The transducer does pick up fish and depth pretty reliably on calm water, and yeah, the convenience of seeing everything on your phone is nice. Battery life is decent enough for a few hours of use, which covers most casual outings. The app is pretty basic but gets the job done.
The real catch is in the limitations. Water clarity matters way more with these budget models - they struggle in murky water or when there's a lot of vegetation. The range is shorter, so you're limited to smaller ponds and controlled areas rather than open lakes. And the durability is questionable; I've heard plenty of stories about them dying after a season or two of regular use. They also tend to have connectivity hiccups, especially if your phone or the device moves too far apart.
So for small lakes and ponds where you just want a basic sense of what's down there without spending serious money? Yeah, grab one. You'll get useful feedback. But don't expect the reliability or detailed mapping of a Garmin. If you end up getting really into fishing and want something that'll last and handle tougher conditions, you'll probably upgrade eventually. The budget models are good for testing the waters without committing much cash.