Gosh, I'm on the fence with this unit, and disappointed in myself that I jumped the gun before the Amazon reviews were in. If you buy this unit, by all means purchase from Amazon or another reputable retailer with a good return policy in the event you are not satisfied with its performance. I'm reserving the right to upgrade this 3-star "It's OK" review at a later date, but for now (after a couple weeks of ownership), this is where I'm calling it.
I bought the GMS120 to replace a couple of older Zircon units that proved OK on drywall over fir framing, but I've moved to a more challenging climate of plaster walls, some with wire mesh.
But first, on drywall, the unit has been hit or miss. I broke it in on a wall that's open on the back side (so the framing is exposed), and I'm not getting the precision I've come to expect with the center-finding Zircon unit I own (which has also been hit-or-miss). Where the Zircon projected a finely focused LED beam along the calculated stud center, this Bosch unit gives a LCD "bar graph" display plus a 1/2" wide lighted circle that changes from red to yellow to green -- so the user is responsible for tracing a circle, or drawing a crosshair inside the circle, to note this location. The indicated center of the stud is often not repeatable, so it takes a number of back-and-forth scans to get a good feel for where I should be drilling. And on the drywall over metal stud walls at work -- big disappointment; no usable readings.
On plaster, I've been confounded by this unit's inability to locate a stud correctly. The instructions recommend placing paper or thin cardboard backing behind the sensor to assist with the uneven thickness / density of plaster walls, but the 1/4" lath appears to be too much to overcome. Never mind when chicken wire is in the mix. I'm going to keep playing with this and find a hopeful solution, as according to the reviews out of various European publications I've read, this shouldn't present an insurmountable challenge.
Worse yet, the current sensing function seems to be entirely non-functional. I've traced over known, energized wire locations with nary a *beep* from the scanner. Another reviewer states that this current sensing function is active in the wood scanning mode (I stated contrary previously and stand corrected), but I've yet to receive an accurate current warning from this scanner, regardless of mode selection). If I recall correctly, the Zircon stud finder sensed electrical current when in stud finding mode, so where two passes are required with the Bosch, the Zircon would somewhat reliably call out a live wire while in the process of scanning for studs. This function works if I place the sensor directly on live Romex or even a power cord plugged into a wall socket, but once drywall is sandwiched in the middle, readings disappear.
And the LCD display... what was Bosch thinking? There's a lot of info on the display, and it's backlit (user selectable on/off -- yay!!) but the viewing angle turns to zero if I hold the scanner just a FEW INCHES below eye level -- ZERO, as in INVISIBLE -- it appears the thing powered off. Higher than eye level, no problem, but lower -- even with the backlight on, it's completely unreadable. Seriously, Bosch? This may not sound like a big deal, but consider for a moment how you hold your arm while using one of these gizmos. I can't speak for the wider user base, but I feel most natural / comfortable with it at chest / neck height, exactly where the display goes blank.
The scanner comes with a rather thorough multi-language instruction manual, which shouldn't be necessary for something so simple. Nevertheless, it's true to Bosch's tradition and appreciated. It also comes with a tight fitting canvas-style case which truly could be just a tad bigger.
I'm holding out hope that I'm missing something important here and I'll grow to love this unit, based not only on Bosch's reputation, but on the stellar reviews I've read in various trade mags. Honestly, I'm not seeing this as being any better than the scanners available from Zircon, and the half-viewable LCD display is an absolute disappointment.
My last stud finder purchase was 6+ years ago, and after reading some great "pro" reviews about this unit, I was fully expecting that the state of the technology had greatly advanced versus the Zircon that was already in my tool bag -- especially coming from Bosch! I'm in the process of contacting Bosch customer service to see if perhaps I have a lemon, because again, my experience has been nothing like the reviews I've read proclaiming the GMS120 to be the best in class.