Hi there.
I bought mine after trawling around for a 5 Touring or A6 Avant to replace my 7. I also briefly looked at E estates. Like many others, I decided that the stratospheric money being asked by the German dealers, and the lack of availability of 5s, meant that the 300 was a real value for money choice. I've not driven very many miles in the 300, but after some mixed and motorway driving, I think that, blindfolded, I'd be hard pushed to tell the difference between my 7 and the 300. If anything, the 300 has weightier steering and a firmer, more positive ride. The only downside is that the 300's transmission is not quite as good as the 7's, with a noticeable lag when pulling away, and a lack of change down on the overrun.
The diesel is better than I had hoped, with similar performance to the normal setting on my 7, and only a bit less shove in the back than the 7 on loony mode, which was never very useable for more than a minute or so at a time, as corners would get in the way.
Inside, the 300 is just as comfy as the 7, and a whole lot more visually appealing, (door panels excepted), than the 5.
That aside, the 300 certainly has the visual "wow" over the ugly current 7 and the anonymous 5 and A6. Currently, I have not experienced a single "I wish I'd gone for a 5" moment.
Go on .... go furrit. You'll enjoy it.
cheers
Z