Did you know that the built-in reporting wizard actually supports M:M relationships? I was surprised to note that some CRM developers didn't know this but yeah, the reporting wizard does support N:N relationships and group by clauses using those; it is pretty straight forward, just launch the reporting wizard (new>report>Reporting wizard) select your primary record (e.g. Account) and on the related record dropdown select the N:N relationship (e.g. Leads), the rest of the flow is quite simple. If you group by, lets say, the name of the account, the reporting wizard will generate a nice report showing all the related records for any given account.