I’ve been using a Dakota 20 for over 5000 mainly off-road miles now & it’s brilliant. Mine’s been through the mill a fair bit & never let me down, other than the original bike mount that for some reason didn’t hold the unit well enough, causing the gps to bounce down a flinty descent on more than one occasion. A 2nd mount has been working fine now for ages. The unit is tough as old boots, batteries last well (less with HR monitor but still good enough for around 7 hrs or so), though some OS mapping I downloaded did sap the battery a fair bit. AAs are great for multi-day & looong rides though, which is the main reason I went for that one.
I’ve been using the free Open Streetmaps instead of OS mapping, which has been fine in England & Wales at least.
I’ve also now got a Garmin 800 for use on the road bike as the Dakota can’t talk to a Powertap, but other than that it’s great.