I've been using the Speardiver Composite Speargun Reel on my railgun now for long enough to feel like I can post about it. The reason I say long enough is because the other reels I had worked well enough too, at first.
Mainly what I am comparing it to (given my experience) are the omer match reel and more importantly the Spearmaster/Mako and Rob Allen Vecta composite reels.
The Speardiver reel has the same capacity as the Spearmaster & Rob Allen @ 70m and 20 more meters than the Omer match at 50m
Speardiver reel
The only reel I have a problem with is the Mako. It was less than a month after having it (and yes I always thoroughly rinse my gear) before it started seizing up. The drag knob would move independently of the top screw and would become, at the most inconvenient times, unable to be adjusted. The Omer match reel had a similar inconsistency with its drag's function.
Furthermore the Speardiver reel has a smoother spin as the spool rests on a stainless steel axle encased in a plastic sheath rather than loosely on a bare metal axle.
You can see my gripe with the Spearmaster reel in this vid around 1:22 I often had to yank line out while ascending yet the drags wide open.