Well as the owner of a Sony 18-250mm lens, which is in effect a rebadged Tamron 18-250 with some minor reworking, I can tell you it's an excellent lens overall, with very good capabilities and results throughout the range, and most importantly, with an incredible range in one tiny package. There are very small modifications to the Sony badged version which make it a bit better, like a shorter focus gearing for faster focusing, and tighter tolerances (lens creep has never been an issue on my massively used and abused 3-year-old lens)...but optically, it's identical to the Tamron. The newer Tamron 18-270mm is more of an evolution than a revolution, so pretty much the same comments apply.
Here's a gallery of quite literally every photo I've ever uploaded from just this lens:
http://www.pbase.com/zackiedawg/sony_sal18250mm_f3563
For an idea of its versatility and usefulness in nearly any shooting situation. Perfect? No, not at any particular focal length - a shorter zoom or a fast prime will beat it at any one focal...but none can come anywhere near the convenience and flexibility.
As to the other comment...I find the all in ones to be an excellent choice for two types of people - 1. for those who have a DSLR but are not really that 'into' photography, and just need the most convenient single portable package with body and lens that can give them nearly any shot they might ever need to do...or 2. for photo enthusiasts and lens collectors who want to include in their collection lenses to cover all bases, and no matter how lovely the collection of shorter zooms or fast primes you've amassed, sometimes a 'travel zoom' or 'all in one' can be an excellent tool for some events, situations, or needs. I personally wouldn't think that because you are a photo enthusiast or professional means you can't also have a great tool of convenience in your arsenal. Hey...I have a heck of a nice Craftsman tool kit in my garage, with nearly every conceivable specialized tool I might need - but also in that tool kit is a multitool or two. I've got hundreds of ratchet heads, but I also have a vice grip. And though I use the specialized tools when I know what I have to do, I also have grabbed the vice grip probably 80% of the time for a quickie job or fast emeregency need that pops up - the convenience of these tools in their ability to allow you to quickly adapt to many different conditions or needs makes them a great addition to any mechanic's box.
Similarly, I have 19 camera lenses right now between two interchangeable digital bodies - split between shorter zooms and lots of primes...but one of those 19 lenses is the 18-250mm. For me it isn't about not wanting to change lenses or have tailored versatility, but having the right tools for the job - and sometimes the ultrazoom is the best tool hands-down - nothing else can let me travel light and compact, with no bag or other lenses, and still shoot wide angle architecture, scenics, portraits, and distant wildlife in one lens. If I'm JUST shooting wildlife, or architecture, or portraits, I grab the primes or shorter zooms that will be best for that specific job.