Benefits of deinterlacers against DVD players with prog scan?

Been browsing through this group, trying to identify what level of improvement can be got out of deinterlaces/scalers when used with reasonably high end kit like my Arcam DV27A. I'm still a bit lost!

In general, what level of kit do you need to use before you can actually obtain a picture quality that makes you think "this has been a worthy acquisition" compared to the prog scan output of a device like the above?

Can the scaling alone make such a difference?

 

IMHO and limited experience it's the de-interlacing rather than the scaling which determines PQ ... there's far less involved in scaling, meaning fewer processes which can be better in one implementation than another. In the area of de-interlacing the range of quality seems quite large, I personally don't think I'd see any significant difference in quality of SD material using a dedicated device like, say, a Lumagen or iScan over that of a good-quality player like my 868.

One advantage of a dedicated scaler like the iScan HD is that they can produce a range of output resolutions, whereas players tend to be limited to a very few. If the iScan had HDMI it would be have been interesting to me for that.