"It sucks"
"It rocks"
All a matter of perspective. Anyone expecting it to generally equal or outperform a dual-speaker setup is going to be disappointed. Versus other single-speaker devices, it's lesser volume than some, more than some.
Coming from a Nexus 6 with 2 speakers, it's a step back in volume, but I find the audio quality to still be very good. I still can't fathom why they didn't go with a stereo front speaker setup as that's what they'd negotiated with the prior two Nexus flagships, and they certainly have the space (the bezels on the XL are actually larger than those of the Nexus 6!), but I can live with it as it's fully loud enough for speakerphone purposes or listening to music, etc.