For those that have successfully gone off-grid, how many watts?

Depends, if you are going to build a small solar panel to power a 3v, 4.5v, 6v radio then buying individual "cells" will work.  If your going to build panels to power your home "cell by cell" then you are dreaming.  I had the intention (a long time ago) to build a solar panel by buying single solar cells once a week (had $5 allowance).  The 3rd week the store ran out of cells, when the 5th week came there were more cells but they were different (poly crystal instead of mono crystalline).  If your thinking about saving money then buy the panels - you can get 30% back from the government.  Type in "used Solec panels" in your web searcher for a deal on slightly used panels (30% off -not a deduction).

My perspective on this is that if your local utility has net metering (power company buys back the excess power you produce), use it! This eliminates the need for expensive batteries since you use the power grid as your storage device. During the day when you are out (and not using much electricity), your PV array is selling its excess energy to the grid. During the times you are at home, any 'excess' energy use that exceeds your PV array capacity is then purchased from the utility grid. An optimum system would zero out (in dollars) what you save with selling excess power and buying needed power. If you over produce, your investment in the PV array is not being recovered since the utility will not pay you enough to justify spending $4/watt on extra panels. If you under produce, you still pay your utility bill but at a reduced cost.
Of course you can still add batteries if you want to go through blackouts (at night!).
Another thing to consider is peak loads. An electric water heater draws 4500 watts when heating (typical). This is why replacing water heating with direct solar is such a deal. The next big thing is the oven which can also use periodic, but large amounts of electricity. Also consider start currents for motors (A/C, refrigerators, etc). Any 'off grid' systems will have to deal with the 'worse case' combinations of these loads (in terms of battery capacity). If you are net metered, the utility supplies you with plenty of power to cover these peak loads.
Being totally independent of the grid has its charm, but requires a lot of thought on smoothing out the loads so as to reduce your investment costs and maintenance costs.