I use Merrill Edge for my Roth IRA, mainly so I can get free
premium checking at my local branch. A couple years ago
I moved my other accounts to Etrade to qualify for a $2500
bonus and will probably consider moving them again to a
different discount broker for a similar bonus in the future.
Most of my experiences with Ameritrade, Merrill, Etrade,
etc. are pretty similar. I haven't seen major differences
between them, but I'm a buy-and-hold investor. Very
few trades per year (maybe 2 or 3), so the slight difference
in trading fees isn't material.
I just recently opened a Schwab account for the following
reasons: 1) $100 bonus; 2) Excellent service, especially in
using a debit card internationally; 3) I'll be able to convert
200K+ Membership Rewards points from American Express
into my Schwab account at $0.0125 per point.