Multi tools come in 3 categories,keychain tools, general purpose full size tools and full size tools targeted to specific users ( gun repair,bike repair and general purpose with extra tools).
If you need a trade specific tool you will more than likely locate it at a specialist dealer or a shop specific to your needs( gun dealer or bike retail shop).
Then look at your requirements, how handy are you in reality, if you are capable of ding a variety of repairs opt for a full size multi tool,if not keychain carry may be the best option. Good brands to look for a re Gerber,SOG,Victorinox and Leatherman,avoid cheap clones they may let you down and come with no =back up.If the full size tool is part of your every day carry then consider whether you will wear it on your belt or carry it in a bag or backpack,some can be rather heavy to say the least.Pocket carry is generally not an option as the tool weighs you down and pockets gape from the weight.A good blade has proved invaluable around the camp for cutting meat,opening packets and even peeling fruit.
Look for a tool that has a decent sized screwdriver as you will be surprised at how often you will use one.The same applies to the scissors you will use these more often than you think. Choose you pliers carefully as a long nose option may be more useful ,however often they fail when it comes to heavy work. You will also use the bottle and can opener as well as the awl,you will actually be amazed at the multiple functions that you can get out of a multi tool. Some even have nifty corkscrews and mini screwdrivers( think spectacles) . Tell us about you choice of tool and any special experiences you may have had at.