One thing many people new to photography (who are looking to buy a camera) don't realize, is that any system you buy into... you are basically going to have to stay with that system. After investing money in a body, and lenses (that only fit that body)... as well as any other system specific accessories they may purchase, it is very hard to justify changing to another system (as well as very expensive).
Just assuming that as they progress.. they want better bodies and lenses, and may even eventually decide to go PRO... they should pick a system that is capable of supporting that future! Canon and Nikon both have extensive Prosumer and Professional lineups for Bodies, lenses and other need accessories. They are also compatible with almost all third party accessories and lenses.
Sony has neither a PRO line, nor does Sony have the width of compatibility with many third party accessories that Nikon and Canon enjoy. Buying a Sony (and locking yourself into that system) can cause issues in the future, should one decide to go beyond being a casual hobbyist photographer. This is one of the biggest reasons that I feel Sony is not a good choice for a serious beginner..... we should always plan for the future, after all!
+1. As anti-hipster as it sounds (so you may want to swallow your Starbucks Frapalatepaidtoomuchforcrappycoffee grande so you don't spit it our with what I'm about to say!) With many things, it's often better to stick with the popular, road well traveled brands. There is lots of expertise, lots of available third party accessories (Why on earth would a hobby photographer like me spend $150 on a remote shutter when I can get a cheap one off of Amazon for $10?), long history of accessories and bodies, and literature and articles galore to help you get the most out of your camera. Maybe in several years Sony will fit that bill, but right now they don't.
Are they good cameras? Absolutely! Do they have a market? Absolutely! I know LOTS of people who bought a DSLR 5 years ago and have been using the kit lens ever since, shooting in full auto mode. Hey, if you've got the money, why not? A good DSLR in full auto mode will blow away a point and shoot in full auto mode! So if you are in that market, a Sony DSLR can really be the way to go.
But if you are on this forum, you are
probably not in that market. That means you
probably intend on upgrading equipment, and you
probably like to try new things, learn new tricks, and try to get the most out of your camera. Can that still be done with a Sony? Again, yes. BUT, you'll have less people to learn from, less literature available to help you learn new things, and a much more limited selection for upgrades. You may even hit a point where you finally say you want an incredible, high end camera, one that takes razor sharp photos, but guess what? Sony doesn't have one of those. Now, buying a Sony doesn't bar you from buying a Canon 1DX or a Nikon D3X or D4, BUT, if you had already been using Canon or Nikon, then you already have a selection of lenses and accessories that will work with your substantial new investment. If you were using Sony, then you have to invest in all new glass and accessories.
BUT, that's not everyone. I regard Sony DSLR's as among the best solutions for a point and shoot photographer, or for a DSLR photographer who doesn't intend to spend a lot of money at this. There are those who shoot a DSLR and will likely never spend the kind of money others do on equipment, and could safely invest in a Sony system and never regret it one bit. You just have to determine who you are. Are you in the Sony-Nikon-Canon market, or just the Canon-Nikon market? (Or even just Canon OR Nikon, as might be the case if you've inherited some glass!) My Grandpa could probably fit the Sony market. Before he retired he was a professional photographer (of sorts, he did marketing, and did most of his own photo shoots). He also did a lot of hobby photography that would just blow you away. He got away from it though when everything shifted to digital. But, he's back at it now with his T1i Camera. He will likely have that T1i forever (He had his Minolta camera body for over 30 years!) unless it breaks. He buys a lens every couple of years, etc. In fact, the only reason he went Canon is because I shoot Canon and this way, we can share lenses and accessories. We like to hike and shoot together, and it's easy to carry all the lenses in one bag and just grab what we need for a shot, works great!
It's like anything else. You need to be honest about who you are and what you want to get out of something, and invest accordingly. If you have a 120 mile commute, a medical condition that limits your lifting ability to 15 pounds, and have never had a desire to own anything that goes on a trailer in your life, then a lifted pickup truck is probably not the ideal solution for your next vehicle. Is the pickup bad? No. Will the pickup fail to get you to places that a small sedan could get you? Certainly not. But you'll waste money hauling around advantages that you'll never use, with a vehicle that is more expensive and uses more fuel. On the flip side, if you own a construction company and routinely haul heavy equipment and supplies to muddy job sites, then a sedan would be silly, as you would have to constantly rent pickup trucks (or buy one in addition to your sedan) to get to work. Owning both is silly too, because the cost of the sedan throws away any savings you might have on the rare occasions you are able to take it instead of the truck. This argument is much the same. Why waste money on stuff you'll never use, OR, why throw away money on something you'll need to completely replace in the future?
I aplogize about that, but it is what it is. I do everything my self including the website itself, and just can't afford to pay anybody to proof read my articles. Spell check and grammer check obviously miss stuff, and clearly "writing" is not my strongest skill set!! 
