I've had a Revolut card and account for several years, and have had very positive experiences. I use it a lot for online purchases (especially Sterling), I have used the card in bricks and mortar shops and ATMs abroad. I save small amounts on each transaction because of the exchange rate.
The most common issue is the "service drop", when a transaction won't go through because Revolut are having problems. This happened me once when I tried to pay for a B&B in the UK. Payment wasn't authorised, but the debit from my card was frozen, which left me low on funds temporarily. Revolut recommend that you should always have an alternative source of funds.
Edit: As Brendan said, I don't keep too much on the card/ in the account, just top up when I need to.