As an ex-mechanic, I have a pretty large selection of tools and have tried just about every brand there is. Here are my thoughts:
For the professional mechanic who's constantly using his tools and running back and forth to his tool box, you simply have to have quality pieces. Snap-On makes great sockets, ratchets, screwdrivers, air tools (3/8" ratchet, 3/8" impact, air drills, die grinders) and wrenches (for the most part). Snap-On torque wrenches are second to none, as are their pry bars & gauge kits (compression, leakdown, etc).
Mac makes great boxes (Maximizer rules!), air tools (their 1/2" Impact is awesome as is their 1/4" ratchet) and cutting tools (side cutters, etc). I also prefer Mac electronic tools as they use Techtronix. I have their handheld scope/DVM, and an assortment of other electronic tools (timing lights, etc).
Matco has some cool specialty tools, and I really like their pliers and needle-noses. They had some great diagnostic tools at good prices, too.
Most importantly is the quality of the tool salesman. We had a really bad Matco dealer (went out of business after a while), so I rarely bought tools from them. Our Snap-On & MAC guys were great, so that made a huge difference.
Sears makes some great tools for the weekender, but there's some stuff that you just have to break down and spend the cash with the "big guys" for. I started out with a Craftsman box, but it wore out after about 4 months of everyday use. I bought a Snap-On box and then traded it for a Maximizer top/bottom combo. After I got the Maximizer, I fell in love...there's no beating a quality tool box.
Long and short of it is: buy what you can afford, and upgrade when possible.