In reply to James Malloch:
> Has anyone used one before who's able to comment on it's pros and cons?
Oh yes. The office gym where I once worked had a lat machine with a non-detachable handle, below which I attached a one handed grip on a sling. For comfortable one-arming rather than finger training. I needed stitches when the main handle spun and I pulled it down onto the top of my head as hard as I could.
This might not happen to you if you're mainly going to use if for static holds. But a better bet if you've got access to enough free weights, kettlebells or whatever is to make a little "no hang" rig. Bit of wood with a hole drilled in it, crimp screwed onto to it, sling or bit of rope through the hole - and use this to one-arm deadlift some weights. Easier on the shoulders too. A small campus rung doesn't cost much and makes a good training crimp - good size, nicely rounded etc.
Or just hang your portable fingerboard from a fixed object such as a pull-up bar?