It all depends on one's ears, some will say not worth it coz have higher expectation, but for some others good enough. I for one used to use big 12s from top US brands like JL Audio in the back either in free air or in a custom box powered by hundreds of watts of US amps, but now I just use a pair of China 10' underseat powered subs. Sure the under seats will not generate as much kick and as loud as the huge 12s but now I'm quite content already with those cheap underseats, enough to bring just a bit more live and punch to the music without breaking my bank. I guess as I get older my priorities and expectations already shifted as now I don't need to have a system that can shake my whole car! So in short just have listen yourself first if good enough or not, then only decide.

Which underseat sub are you using and how much did you buy it? How much also the ported sub? Have you factored in the price of the power amp to drive the ported sub? Because underseat subs usually already come with built in amps, so to compare with big sub boxes, you need to also include the necessary power amps (or compare with built-in powered sub boxes).
Of course the main limitation of underseat subs is physics, more exactly space as it only has very limited space to work with and therefore cannot move as much air to generate big bass as regular sub box.

Underseat subs are subs to be placed under the seats, they mostly also already active type/have built-in power amp.
Active sub is sub that includes built-in power amp in the box/casing. It can be may different shapes/designs: underseat, tube, or big box.

Bagus depends on one's expectation given the money spent.
I'm also using a pair of MBQ but 10s', bought around rm500 each all installed. I would say it fulfill my needs to just improve the bass sound of std audio but of course it would not match traditional big sub box. My criticism of this thing is the settings of the power amp gain is not good quality, can have high humming distortion on higher gain settings so it was difficult to set up just right.