As I recall, the UK didn't manage to pay the US back till the early oughties, but I don't think other European countries that received post-war US aid had the same problem.
This fact seems to be a very big deal to some British people I've spoken to, but I don't remember it being brought up in AP US History at all. I'm skeptical of how WWII is covered by the basic history classes in either country.
The fact the debt took so long to pay off is actually a sign of the generous terms the UK was given (at least, from a commercial point of view). The main loan, taken out in 1945, had an interest rate of 2% and was due to be paid in 50 instalments, starting in 1950. Normally long-term loans come with higher interest rates (because the lender has to wait longer before they get their money back), but in this case the rate was so low, that effectively it was often negative.
The UK was also able to miss its payments in some years when the exchange rate was unusually bad, which is why the loan was repaid in 2006, rather than 2000.
When the loan was made, it was a shocking amount for a country that was struggling to support itself. In his negotiations with the US, Keynes had pushed for a grant rather than a loan, and was disappointed by the response. Had the postwar years not been so prosperous, the loan could have caused more problems. Fortunately, strong growth meant that it fairly rapidly became a relatively small item compared to general government finances. (I haven't seen specific numbers, but I don't think it was a significant issue after the 1950s.)
This BBC article gives a short but effective summary.