Some ideas are so simple, you just have to wonder why nobody came up with something similar earlier. The concept of a modular mouse is one such idea. Unless my memory betrays me, I don't think this has been done before, and if it has, I'd be surprised if it was executed this well. For the M42, Xtrfy not only includes two interchangeable back covers, but also provides the necessary files for 3D-printing. With those and some tinkering, even an ergo-like shape could be created on the M42, or any other shape one may think of. But the M42 already provides a great deal of customization even with just the two included back covers. Depending on whether the high or low-profile back cover is used, the M42 feels radically different in hand. I was quite surprised by how much the back portion of a mouse actually contributes to how it is held as a whole, and more importantly, this concept is executed pretty much flawlessly. Both back covers are well-built, and the mechanism to install them is convenient yet firm enough to sustain frequent tinkering. In fact, the whole mouse is built very well, with no flexing, rattle, or wobble to speak of despite a lightweight construction of no more than 58 g.

The M42 performs incredibly well even if looking past the modularity. Compared to the M4, not much has changed, which is no doubt a good thing. In fact, one of the only flaws of the M4, the side buttons, has been addressed. While still not perfect, the side buttons are much improved, having less travel and a firmer click. Generally, button quality is great, including the scroll wheel, which still provides firm and tactile scrolling. The cable is the same as on the M4 a year ago, and while quite a few mice with more flexible cables have come out since then, I feel that this one is still flexible enough. The mouse feet provide excellent glide, and Xtrfy added a set of replacement mouse feet, which is always appreciated. As for the sensor, PixArt's PMW3389 is still implemented pretty much perfectly. There is once again no smoothing across the entire CPI range, which sets it apart from most 3389 implementations. I'm quite pleased to see CPI deviation this low as there is no software that would allow adjusting any off-target steps. Thankfully, the Xtrfy M42 allows for extensive on-device configuration, in particular of the many RGB lighting effects, which look as nice as ever. Lastly, click latency is nice and low, too.

All in all, Xtrfy impressed me once more with one of their releases. In terms of pricing, the Xtrfy M42 does quite well if we look at the competition. The ROCCAT Burst Pro performs similarly well, comes with optical switches, and has slightly better build quality, at $59.99. The VAXEE ZYGEN NP-01 goes for $59.99 as well and shares the no-software approach with the M42 while performing similarly, but is of slightly worse build quality. The Endgame Gear XM1 White has the M42 beaten when it comes to cable flexibility, but otherwise scores similarly in terms of build quality and performance, again at $59.99. Lastly, the Razer Viper Mini for just $39.99 is a bit behind in terms of the sensor, but comes with optical switches and good build quality. In any case, the Xtrfy M42 gets our Editor's Choice and Innovation awards from me.