As of firmware AXP2CN1733AR the HP T120 is much better, see here

As the picture below shows (which is from large to small: AdobeRGB, sRGB and HP T120 on HP Universal Gloss Paper), the HP T120 produces roughly 70% (by eyeballing the image in top view) of the sRGB color space. That is not very impressive. For instance, the Canon iP100 I own too (that also has a four-color ink set) has a much better reproduction of the sRGB profile than the HP T120 has on any photo paper I have printed on, as illustrated by the screenshots below. In top view, you could argue that the Canon iP100 and the HP T120 are close, but the lower screenshot reveals the much superior black reproduction of the Canon iP100.

Below: from large to small, AdobeRGB, sRGB and HP T120 on HP Universal Gloss Paper.

Below: top view of a render of the ICC profiles of the Canon iP100 on HP Advanced Glossy Photo Paper (a profile I made myself) and T120 on the same paper (T120 is the inner render).

Below: same as above but shown from the side.

To give you another comparison with a dedicated pro printer, I show screenshots of renders of ICC profiles of the HP Designjet Z3200 versus the HP T120. The Z3200 is considered to have a broad gamut and solid black reproduction for a pigment printer due to its gloss enhancer.

Below: top view of a render of the ICC profiles of the HP Z3200 on HP Instant Dry Satin Photo Paper and T120 on HP Premium Plus Sation Photo Paper.

Below: same as above but shown from the side.

The comparison with the Z3200 ICC profiles shows that, altough visible in the above shown ICC renders could seem marginal, the differences in practice are enormous. I have owned the HP Z3200 and I can tell you that the blacks it produces are much deeper than the blacks that the T120 can produce. Further, the gradients (for instance skies) are very smooth with the Z3200 whilst the T120 shows heavy dither signs (in part also due to the grain in lighter colors that the T120 produces (see this post on grain produced by the T120 and how avoid it as much as possible).

UPDATE 5-2-2018: Since firmware update AXP2CN1733AR the HP T120 is much better, see here.

The limited gamut results in heavy dithering effects and abrupt co.or transitions in gradients in your photo.

Below is a close up of a wedding dress showing this effect, followed by a close up of brown dust in front of a blue sky and finally a typical sky gradient (click to enlarge).

The above results may scare you, but in practice it is possible to achieve fair and sometimes even reasonably good photo prints with the HP T120. It does, however, mean that you have to soft proof your photos in Photoshop or the likes prior to sending them to the printer to make sure you do not have or limited the bad gradient reproduction as shown in the examples above. That soft proofing is mission critical to getting acceptable photo prints out of your HP T120, is shown best by the below screenshots.

Below: Datacolor sample image as shown on my calibrated monitor (in other words, this is what I see on my monitor before turning on soft proofing).

Below: Datacolor sample image as shown on my calibrated monitor with soft proofing turned on for one of the HP Photo Papers I calibrated (in other words, this is what the printer will print based on the image above!).

Not only the blacks are heavily affected, also the color reproduction is much more dull in the soft proof version. You have to compensate for this in soft proof mode in Photoshop or the like to avoid dull prints rolling out of your T120. Also, be sure to experiment with the color rendering intent during your soft proofing (for each photo file). I get good results with "absolute colorimetric" (good for reducing bad gradient reproduction) and sometimes with "saturation" (for photo's with little gradients). Be sure to try it yourself. The right rendering intent can make all the difference in the world with the T120.

Next: grainy prints and how ro reduce grain somewhat.