You're better off doing custom scripts. MERGE can, and does, function. But, it's not very tuneable. You get a single execution plan for the INSERT/UPDATE/DELETE regardless of which is used to compile the plan. It makes it very inflexible. If you need the ability to tune the query, you'll have to customize, so why not start there. It's not much more work, about the same really.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning