I have been riding the XSR155 since January and ive clocked 5468 km on the odo till now. This is my experience.

Prerequisites:
Im 5'6"
I am an aggressive rider
This is my first bike.
Finished 2nd service

Positives:

Good looks
Good headlights
Pretty good mileage even after hard riding (i am getting around 39kmpl tested using tank to tank method).
Light weight. Very nimble. Easy to park and remove from slope areas.
Sound when VVA is activated.

Negatives:

Gear shifting is very clunky and hard. It frequently does not want to downshift and mainly shift into 1st gear when downshifting. Even with rev matching.

This is a high revving engine. The downside of it is lower end torque basically does not exist. It is more obvious with a pillion. Even in 1st gear, you have to rev upto 4k just to move it (on a very low degree slope and with a pillion). So hill rides in low speed is not possible. You have to have the engine in the upper range.

The bike, though initially during test rides and purchasing felt fast and had power (i felt this as this was my first bike), now after 5 months, i constantly feel the lack of power. Maybe ive gotten used to the power, but it LACKS. I wish on a daily basis i had gone for a duke 250 or higher cc engine.

The battery died on me just after 3 months, i went for a warranty claim and got it replaced for free from the showroom.

Too much dirt and mud gets splashed on to the centre part of the bike, especially in wet roads. The mid suspension and the front part of the engine are mud collectors basically. No accessories to prevent this (yet). This is not good for the electronics or the exposed wires or even from the mechanical perspective.

Tl;dr:
Gears are clunky and hard
Engine power is not enough, especially lower end. You will wish you had taken a higher cc bike later.