In the United States as well the enforceability of such agreements is dependent generally on state law requirements. The relevant areas are what type of geographic restriction and the scope of the restriction (the more closely tailored, the more enforceable). Some states however, California and Massachusetts in particular but the list is growing, restrict the use of non-compete agreements almost completely. There is a strong concern about prohibiting individuals from earning an income.
If you are concerned about top talent leaving, rather than restricting them for leaving it may be more effective to incentivize them to stay. Retention bonuses, key assignments as part of the integration team, career development may all help you reach the same goal.