We've had dexters for about 4 years now; we chose them because they're about the only breed we can accomodate on our very small smallholding.
If I had my time over again, I'd choose to keep something a bit less demanding. Lions, maybe.
Dexters are escape artists, like goats. They can scramble up banks, wriggle through tiny gaps in hedges (they'll be big gaps by the time they've finished) and demolish any fence known to man. They walk through electric fences like they aren't there.
Because we've only got 3 cows and their offspring, they're very wary, practically wild; catching them up for TB tests, ear-tag checks, Bluetongue jabs &c can be something of a challenge. My only previous experience was with black-and-whites, so I wasn't at all prepared...
They're wonderful animals and the beef is superb, but I would suggest they're not for beginners. Wish I'd listened to my neighbour, who told me to get a Jersey.