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In my situation I went it alone so that I had total freedom (to a point - always accountable to the client :) - so a partnership wouldn't have suited.
As for legal factors, nowadays a strong contractual partnership can cover most situations. Just make sure there is one!
Sometimes, a partner brings some major skills or resources to the business and it's worth sacrificing your own absolute control and sharing the profits. Similarly, you may have gaps in your management ability or skill set that you think could be offset very well by another individual. Far better to have half of a huge cake than 100% of nothing.
If you evaluate the situation and decide that it really is worth bringing someone else on board, negotiate the best deal you can and be sure to have a comprehensive contract drawn up. You'll want to be covered if things head south. What if your partner wants out? What if he gets hit by a bus the day after you do the deal? What happens if he fails to perform as expected, or if he simply decides to put out far less effort than you? These are all real possibilities, so be sure your attorney has you covered.