Having switched to cajon from kit as my main instrument a while back (i've only recently started behind a kit again) - I got to play and sample a few...
I had an adjustable snare one. Mappa Burl I seem to remember
It was never that good, as sometimes the snares buzzed uncontrollably no matter what I did.
Someone then kindly put a kick pedal through the front head, so whilst waiting for it to be repaired, I was really surprised at the build quality inside.
Lots of rough edges, poorly cut angles... Put me off buying another.
Personally I'm not a fan of them, BUT, the sound is very compressed which is why I think a lot of people use them in place of a kit.
Tried various models, never found a nice one.
Pretty standard, as you'd expect from a percussion (rather than drum kit) manufacturer. Warm bass, and more of a ringy snap, which I like.
More generally, I'm wary of the more gimmicky ones; 2 (or more) tapa (batter) surfaces, wedge-shaped, extra large, extra bass etc.
Cajon's put out a helluva lot of bass, especially when mic'd, so you don't need any more of that.
The ideal for most is to separate the snare and bass tones, i.e. as little snare as possible on the bass tones. Many get around this with not having much sustain on the sound (e.g. Schlagwerk).
Ensure it has some snare adjustment, though personally I don't see the need for on-the-fly adjustment (pedals, knobs, levers...).
If it's for anything more than the occasional acoustic gig, I'd look at spending a bit more (£300-400). As with snare drums, there's a sudden leap once you spend over £150.
I play a Pepote Estudio (Wembley Drum Centre stock them), and LOVE it.
They make more entry-level ones, and if you can try one, I'd recommend it.
Interestingly, they now white-label some of them to Meinl as "Meinl Artisan" cajons.
Sela also get good reviews but I've never found one to play on.