I obtained a pirate copy of the OS mapping for my eTrex30 many years ago, and it's been on my SD card ever since. I've probably used it half a dozen times.
As it's a bitmap, it ties you into 'north up' mode (otherwise the text appears upside-down when you face south). More problematic is the relatively low resolution - it simply doesn't show the detail of things like road junctions accurately enough to work out (for example) which exit you need while you're riding round a roundabout or negotiating a compound junction in a town centre. Also, as it has more to render, it's also noticeably slower at panning/zooming/rotating than the vector maps.
On the other hand, it's a real OS map. Second to none for topographic features. And (if you're asking this question) you probably have an intuitive sense of what all the symbols and colours mean.
I concluded that it's quite good for walking in the countryside, and some off-road cycling, but an vector map like OSM or the old Garmin City Navigator was better for road cycling / driving, and probably walking in urban areas (though there are strong arguments for the likes of Google Maps for that).
In reality, what I tend to do is use my eTrex with vector mapping, and occasionally whip out Viewranger on my tablet or smartphone to refer to a proper OS map (usually 1:50k, but I can download 1:25k tiles when necessary) when I want the extra detail or 'big picture' view. These devices have bigger, higher resolution screens and much faster CPUs, which makes for a much more pleasant OS map experience.