I do not have a wide experience with different equipment (I do not change mine very often). My previous speakers (which I still have) are B&W Matrix 802 S3s. I also owned Acoustat 2+2s for years. I do visit hi-fi dealers every now and then for a listen.
The most distinctive characteristic of the Mantra 50s is that they are fast speakers. They have a speed and delicacy which brings to mind planar speakers.
Their tall slender build mimics a vertical a line source to some extent. Imaging is very good. This type of build should also reduce floor and ceiling interactions.
I would say that they are pretty neutral: neither forward nor laid back; maybe slightly warm. They seem to more forgiving of less than perfect source material than the other speakers I have owned.
As far as bass, mine measure pretty flat to about 40Hz in my relatively small 11’x13’ room. The bass may be a little lacking in weight and scale compared to my B&Ws. It does seem to have plenty of punch, though. I also bought a subwoofer (JL Audio) to supplement the bass; I cross it over at about 45Hz.
They are said to be not very picky about amps. They seem to work fine with my Audio Research 100.2 solid-state amp.
I (and my fiancée) find the speakers very attractive.
As mentioned in other posts, there are only a handful of System Audio dealers in the USA (I bought mine at Wilson Audio in Metairie, LA). I think that they are much better known in Europe (there are a good many listings of used, mostly older SA speakers for sale in Europe on the various websites; only occasional USA listings).
I think that the System Audio Mantra series speakers have recently been discontinued. It looks like they will be succeeded by the Legend series. There are also the less expensive Saxo series and the more expensive Pandion series.