Sorry I'm late getting back to this thread, but I just finished a job and had to make a quick stop before returning home.
As for MY experience, I A/B tested the Bose L1 against a pair of Barbetta Sona 32SC powered speakers, which I consider excellent, conventional speakers. I also tested it against a pair of Peavey SP5Gs. Bottom line is, there was no comparison. The L1 was much cleaner, crisp, had loads of bottom end, the setup time was considerably faster, and my audiences loved the Bose sound quality.
When it comes to stereo V/S mono, the audiences, as Diki clearly stated, really don't care. They're there to enjoy the music, not have their heads blown off, party, dance, whatever.
As for who hears it and who does not, there is lots of information available about this topic. Essentially, the stereo effect is heard by audience members that are with an equalateral triangle created by the distance between the right and left speakers. Audience members outside that area do not hear stereo, and depending upon your system, the venue configuration, etc.., the overall sound may be dominated by where they are positioned. Those positioned to the right or left of the triangle will predominately hear sounds from that channel. Most audience members will primarily hear mono sounds. The performer, who is usually situated directly between the left and right speakers, however, will overwhelmingly hear stereo.
While phase cancellation has been reported by a couple L1 users, most have not experienced this. It may have something to do with the summation of the channels and various techniques used to combine the left and right outputs.
Additionally, I've heard that the Bose Model II with the Tonematch produces better sound quality from Yamaha's TOTL keyboards than the older L1. I, personally, have not had the opportunity to test this, but hopefully, I will next January at the next Shreveport Jam.
The great thing about your purchase is if you find one works just fine for you, Bose will be more than happy to accept the other unit as a return with no questions asked. They are an outstanding company, their customer service is second to none, and their quality is unsurpassed.
Good Luck on whatever you decide upon,
Gary