Depends how resolving your system is. If the reviewer only gets 'slight improvement', that means his system is not very resolving, period.
To me, it is a 'must-have'. The improvement will be even bigger when you improve your system further. Bear in mind though that once your system is improved, the importance of components becomes more evident, your USB cable will affect the sound even more than before.
I started with just U18 and the improvement is already quite big. Later on, with addition of clock, the U18 becomes even more a 'must-have' component. Without U18, the soundstage just collapses to being flat. With U18, it is a hologrphic 3D soundstage. Also, do try to use some spikes on U18 at the bottom, the improvement on U18 with the right spikes is quite big, soundstage opens up further.
With U18 and a good clock on X26 Pro, working with proper tweaks on PC as streamer, I can tell you the sound is high end level. I am not comparing to similarly priced components any more, just this weekend I listen to T+A DAC 200, and I think my system has more resolution and sounds just about the same analogue like sound, although of course the owner may not have set up the sound of T+A to top level (he may not know much). I think Esoteric N05X (without clock) sounds worse than my system, that gives an idea the level of sound it can achieve.