This is a subject where most crypto'ers understanding leaves quite a lot to be desired.

First of all, if crypto ever gets near a coffee shop of supermarket, FORGET blockchain transactions. We will not be using 'real' cryptocurrency transactions for day to day purchases any more than banks use 'real' banks transfers for fiat transactions today. Even a 20 second confirmation time is 5 times to slow for point of sale purposes.

Has anyone on here ever bought and sold any cryptocurrencies on a cryptocurrency exchange ? (Such as BTC-e, Bitstamp etc).

Those trades are instant. They don't depend on blockchain clearing time or confirmations, because if they did, everything would grind to an immediate halt. Moreover, the blockchain doesn't remotely support the functionality required for point of sale systems, such as refunds, discounts, reversals, approvals, points reward schemes, cashback - whatever. Those are all handled by payment processors such as Visa and Mastercard who "buffer" the transaction to get the trade out of the way for the customer and then later process it through the clearing banks for the associated accounts (that part may take the best part of a day).

If crypto's ever "get big", that's exactly how it will happen. You'll only use the blockchain to "charge" your Visa / Mastercard account. Spending will be done through accounts (just as on the exchanges at the moment). That's possibly why payment processors aren't too worried about crypto's because they don't care what's "behind" the account - could be fiat, could be crypto. They already deal in a diversity of currencies, so cryptocurrencies are just another number. They won't loose any business.

So don't worry about blockchain clearing times or confirmations. It won't be a problem. A half hour confirmation time will still be 5 times faster than what we have today as far as the "clearing" element of the trade goes (the part the customer doesn't see).