This is one of two lenses I own (apart from the kit lens that came with my a200), the other one being the Sony f1.8 50 prime.
This lens excels as an outdoor single lens solution, having an excellent range that encompasses any two other lenses. The Sony version is based on the original Tamron, the Sony having better Bokeh (and some reviews also put the Sony a little faster in auto focus).
As with all lenses of this optical range, there are problems at the very low and high end, but nothing that can't be sorted out in post processing if it is an issue.
A bigger issue for some may be that the 18-250 is often too slow for low light (for that, read indoors under artificial lighting). Of course, a suitably faster lens with the same range would also be twice as heavy and 10 times as expensive... not really an option (or even an issue) unless you want to shoot rock concerts in darkened venues or high speed sporting events.
Another minor (i.e. 'to be expected') issue is that the 18-250 has no macro mode. Again, you can't have a range of 18-250mm and expect macro as well!
Overall, a recommended buy. Not perfect, but possibly the most versatile single lens solution out there, and certainly the backbone piece of any beginner/intermediates 2-3 lens set (especially, if, like most users, the majority of your photos are taken outside or in reasonable light).
The only other lenses I'm looking for to complete my set is the Tokina 11-16 to take my below the low end of the SAL18-250 (and into true wide angle), and perhaps a macro.
*** Update October 2010 ***
Since writing this review, I now have that Tokina 11-16 and the macro (Tamron SP Di 90mm), and I now own the Sony Alpha a500. Still using the 18-250 as a general purpose lens - if Im out and about doing some general outdoor photography, I pack the Tokina and the 18-250, and perhaps the Tamron as a portrait/macro.
*** Update January 2011 ***
Rumour has it that there will be an updated version of the 18-250 lens out soon. If you are thinking of buying the version on this page, it may be prudent to wait and see if there is a price drop because of the newer version. SonyAlphaRumours may shed more light on release dates if the new lens is not out when you read this.