I bought the A550 in late november and must say I was impressed with the results.

ISO1600 is fully comparable til ISO400 on the A300 on pixel by pixel comparison.
When considering the higher resolution I would say that A300 at ISO400 is comparable to A550 at ISO2000-2500 (not selectable range)

I have not printed anything yet, but I believe you get good printouts with these sizes.
ISO1600 A4 or A3
ISO3200 10x15cm or A4
ISO6400 10x15cm

The camera feels more solid than the A300 and buttonlayout is nice.
Didn’t like the placement of the powerbutton at first, but after about a week I don’t mind any more.
Liveview is near perfect and the LCD is really good. (Skip the A500, the LCD alone is worth the difference.)

I know that some people say that high ISO isn’t important, but living in Norway we have lots of low light days.
Those days you need high ISO and low F numbers or additional flash and short range.

Bottom line is this.
If you don’t want to wait for the A7xx camera, you’ll be happy with the A550.
It is not weatherproofed, but neither of my lenses are weatherproofed.

Compact Flash + MemoryStick doesn’t make any sense in a semi-pro camera.

Why would anyone buy a MemoryStick when CF is superior in everything but physical-size?

Yes I know the 550 and 500 has SD and MS but they are not a semi-pro.

They had CF and MS once before (DSC-V3) but that was before SD became popular.
That camera was somewhat crippled unless you bought a MemoryStick. With CF you could film 640×480@15fps vs 30fps with MS.
It was a purpose-built limitation, not a technical limitation.

And last but not least. Why is this a JPG and not a PDF if it is supposed to be real.

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PS.
I have sold my A300 and Google a lot trying to decide if I should wait for a A7xx camera or buy the A550.
There is a few things I would like different with the A550 but all in all a very good camera.