I have the Harbor freight, the Propointer, and The Pro Find and here is my opinion.
The Depth: I like having a lot of depth just as much as not much depth. It is nice to scan the spot before you dig and know exactly where the item is before you dig it. If the item is deep you have to dig the hole first, you will be trying to find exactly where in the hole the item is. This can be done with low proximity like the Harbor Freight pinpointer or by variable vibrate a beep speed like the Pro Find and the Propointer. The latter two also vary their range based on the conditions present when turned on (initial sensitivity), so if you turn these on in the hole near the target, their range will be greatly reduced and they will act more like the Harbor Freight tool. This is not really too necessary as the variable beep and vibrate speed give you plenty of information about how close you are to the target. The sensitivity button on the Minelab is a nice idea but I have not found it practical in the field.
The Buttons: Harbor freight is notorious for bad buttons as is the Propointer, the Minelab is too new to know if it is robust, though it feels like a solid design and we can all hope. The Harbor Freight has a momentary switch which I Love! Also, I own 2 Harbor Freights actually, and neither button has yet to fail (I have also owned these the longest). The Garrett Propointer is also known to have button problems and mine failed after about a week and a half. It was quickly replaced even after I ruined the unit further investigating whether or not I could mod it with a momentary switch (I could not). So while their button seems to be a weak point, they do have great customer service. Also, don't bother disassembling a propointer as it will self destruct and will never go back together correctly.
The Minelab too has an on off switch and apparently must, as there is a short "boot up" time which is my only complaint with it. When you press the button, it beeps a couple of times and then you are ready to go whereas the Harbor Freight and Propointer are instant. If you start the Pro Find up in the hole the initial sensitivity will often be so low as to render it not functional until you restart it. The Propointer will also do this to some extent but does not seem to be as much of an issue and still works but at a greatly reduced sensitivity.
I have stated that I really like the momentary button on the Harbor Freight. It just makes sense. Instant on, and it's automatically off when you are done using it. You can't accidentally leave it on. It does not beep beep at you when you are in between digging and pinpointing because you set it near your shovel. A momentary switch just works better for me. I would like to see a combination switch that can be half pressed for momentary operation and fully pressed for on off operation.
Volume & Sound: None of these have a volume control. If they did it would be very nice. The Minelab can be changed to vibrate only mode but I think they would have been better off to make the + and - buttons volume controls rather than sensitivity control. The Harbor Freight can be modded to vibrate only or both. The volume can be adjusted on any of these by covering the hole with tape or silicone.
Ergonomics: The Harbor Freight SUCKS in this respect. The other two are a bit too big but doable. With technology what it is today, pinpointers should/could be more like a wand than a backwards flashlight on a stick.
Water proof: None of these are really water proof. Even the Minelab says to cover the speaker hole when washing it. I have seen online where people have waterproofed the Propointer and the Harbor Freight though they look pretty bad afterward though and would likely void the warranty on the Propointer. If you need waterproof then the Vibraprobe is you only real option and it has issues too from what I read.
Temperature: The Harbor Freight pinpointer suffers from temperature issues. As it gets hotter, it gets more sensitive. To counteract this you have to adjust an internal adjustment with a screwdriver. To make this less of a pain, I drill a hole through the case over this adjustment so that I can adjust it without disassembling the unit in the field. Just cover the hole with a piece of tape and be sure to carry a small screwdriver with you.
Conclusion: The best bang for the buck to me, is to buy two Harbor freight pinpointers and make the modifications that I mentioned earlier. You are into $35 and have a pinpointer that works. The Profind and Propointer are neck and neck. I like the speed and feel of the Propointer but the depth and extra features of the Pro Find. The added benifit of being able to precisely locate shallow targets is really the only consideration I can find that makes the Propointer and Profind that much better than the Harbor freight. They are better but for the price and for the functionality you get, I still think the Harbor Freight wins. Since I have all three, which one is my go to pinpointer? Well I have to say that when it is working, I grab the Garrett Propointer.
For you "pinpointer makers" out there. If you want to own the market, make a pointer with these features.
Shaped like a skinny Propointer
Robust button that can be half pressed for momentary operation or fully pressed for on off operation.
Waterproof! (at least to 10 feet but more would open up your market)
Volume control (from vibrate only to ear piercing loud)
The depth characteristics of the Pro Find and or Propointer are both fine.
And did I say water proof?
