International customers (like me) consider Datek as a best brand out there, its much more respected that E*Trade. There are tons of potential customers out there (think of Germany, for them e-brokers=Datek). But E*trade has some pluses too (i.e. its banking services - ATM cards and so on), so E*Trade&Datek will be a killer-machine.
BTW as a datek customer, I'd prefer (no, I hope that it'll occur) E*Trade as a buyer over any other firm, and of course I (as 100% of international customers and 90% of all customers) will leave my account at a new firm.