Really depends what movie you're watching. Buying something like ''28 Days Later'' on Blu Ray is a con. Buying ''The Dark Knight'' or ''Blade Runner'' certainly isn't. With a decent TV and HDMI cable these 2 films are miles ahead of any upscaled DVD and the closest to cinema quality as you'll get on any home media set-up.
The True HD sound offered on BR makes all the difference aswell if you've chosen to invest in a Home Cinema System.
Then again, a film's a film, why would you even bother doing that?
I haven't seen 28 days later (on Blu-Ray...) so won;t comment on that film, but I bet OP hasn't either.
They are correct in that many films benefit more than others. Well mastered ones, that have rich colour and multi channel sound benefit from it a lot more than ones with badly mastered stereo sound, muted colour palettes, and bad camera control/crappy transfer all look as bad as their cheaper dvd versions. And usually miss all the supplements that were on the DVD 2 disc or collector's editions...
Nearly all are better thoug; having around 150 blu-rays myself, the major upgrade is old, rubbishy dvd transfers can be gotten rid of in favor of MUCH better quality blu-ray prints.
I would guesstimate that 95%+ show obvious visual improvement - in colour and detail, over their DVD forebears...