I used my UMIK-1 a few times since it seems like "everyone" has one, like everyone has a SPL meter.
But then I put the UMIK-1 away and never used it again.
I guess it's a good "exercise" to scratch that curiosity itch. But in retrospect, I really didn't even need it to make my bass sound great.
Before you even order the UMIK-1, you could just play around with the manual PEQ for your subs. For example, change the 40Hz, 60Hz, 80Hz, and 100Hz to +3.0dB and set Q=0.5 for each. If that sounds like the best bass you've heard in your life compared to the other great bass you've heard, especially from expensive speakers and subs, do you even want to spend hours on REW and the $100 UMIK-1?
Here's one interesting fact - just because your Bass Frequency Response is FLAT, does not mean you will like the sound of your bass.
Of course, if your bass still sounds bad or not as good as the great bass you've heard before, then you do want to invest more time and money to improve the sound.
REW/UMIK-1 can be a good exercise if you are curious about the frequency response of your speakers and subs in your room - say you've been there, done it. And after you've scratched that off your bucket list, you can put the UMIK-1 back in the box and store in your closet forever (like me).