For a brief moment last week, the answer was clear: the HTC One (M8), which I declared in these pages — without hyperbole! — to be the best Android device on the market. But as I mentioned in that review, Samsung would shortly release its Galaxy S5, the new version of its flagship model (and one of the best-selling devices in the world). Would the S5 stand up to its smaller competitor’s charms — even though it need not be the best to sell?

Like HTC’s device, Samsung’s Galaxy S5 runs the newest version of Google’s (GOOG) Android operating system, 4.4.2 KitKat. And like HTC’s device, Samsung has also chosen to install a “skin” on top of it — Samsung calls its version “TouchWiz,” and it’s a big point of differentiation for the company, changing the overall look and feel of the phone’s interface. (Some people, myself included, view TouchWiz as Samsung’s way of bringing its own ecosystem of applications and services to users in preparation to move away from Android to its own Tizen operating system. Evidence of the start of this transition can be found in the recently released Gear 2, Gear Neo, and Gear Fit wearable devices, all of which run Tizen OS.) It’s a look that’s hard to miss on the phone’s 5.1-inch screen — unless you find yourself distracted by the phone’s faux-leather backing, that is.

Inside the phone is a Qualcomm Snapdragon 801 processor, two gigabytes of memory, and a 2,800 milliamp-hour battery. But none of the phone’s innards can hold a candle (or perhaps a hose) to Samsung’s choice to make the Galaxy S5 “water-resistant and dustproof,” a feature that has been in fashion in consumer electronics as of late. Naturally, I couldn’t let such a claim go untested, and after letting my review unit sit in a bowl of water for roughly 10 minutes, I was able to use the device without issue. No matter how many devices on the market carry this protective feature — and there are now many — I will remain impressed (and slightly nauseous when forced to test it).

But back to the parts inside. With nearly identical internals as the new HTC One, the Galaxy S5 should be a fast device. I found quite the opposite. In testing, the S5’s performance was sluggish at best. It suffered from the same multitasking delay I found on the new One, but also slowed performance when receiving a call — when the phone would ring, I would pick it up and have to wait for the screen to light up before I could answer it — and during general use.

The back of the phone carries a 16-megapixel camera, with a heart rate monitor tucked just underneath it. (More on that shortly.) The phone’s camera software gives you the ability to control more advanced photo features (ISO, exposure) and includes image stabilization, HDR mode, and filters. Like most other cameras on smartphones today, it is capable of capturing 1080p high-definition video. The front-facing camera is a respectable 2.1-megapixels, which is fine for its intended use for video calls and the occasional presidential selfie. In my testing, the camera did have some difficulty capturing photos in low-light environments, but overall I found it to be on par with other leading smartphones available today.

The S5 comes with an S5 Health app that uses the phone’s various sensors to count your steps, track activities, and monitor your heart rate. The heart rate monitor requires you to place your finger over a designated area while remaining still and with little background noise in order to get a reading. My experience with it was mixed. On one occasion I had to restart the S5 after seven failed attempts for a reading in order to get it to work — though when I was able to get a reading, the results were in line with a manual count of my own heart rate.

By adding a fingerprint scanner under the home button, Samsung is able to remain competitive with Apple’s iPhone 5S (AAPL). At least on paper, anyway: In testing, I found the implementation of the feature to desperately lack polish. After registering my thumb, I routinely experienced trouble getting the phone to accept my print. The swipe process requires you to place your finger just above the home button, and then move down in one steady gesture. If your finger isn’t perfectly centered, or you don’t complete the swipe, you are asked to scan again. Sometimes I would get lucky and unlock the device on the first attempt, but it was more common for me to see success on my second or third attempt. And if you’re planning on using the reader when holding the S5 with just one hand, forget it: Between the size of the screen and the requirement to swipe ever so perfectly, the entire effort is an exercise in frustration.

As for that battery: It was impressive during my testing. I was able to consistently see 36 hours of use from a single charge. This is remarkable considering the added fingerprint sensor, step counter and other TouchWiz-specific features. On top of that, the Galaxy S5 includes two different power-saving modes to help extend the battery.

Should you buy a Galaxy S5? That depends on whether you have a Galaxy S4. The new model is a minimal upgrade over the old and with the good (camera, battery) comes the bad (fingerprint scanner, heart rate monitor). Make no mistake, Samsung’s new Galaxy S5 is a formidable phone by most measurements — but the sum is no greater than its parts. Why Samsung’s execution lacks for its most marketable features is, for this fair reviewer, a real head-scratcher.

“Logged In” is Fortune’s personal technology column, written by Jason Cipriani. Read it on Fortune.com each Tuesday.