I bought one of these almost a year ago.

Pros:

Very lightweight

Very compressible and portable

Welded construction means very few stitched seams and therefore minimal egress of goosedown from the jacket.

Nice overal quality of manufacture.

Doesn't make you look like a blimp when you wear it (both due to the welded construction and due to the efficient use of down, rather than highly-stuffed).

Cons:

The DWR coating lasts a few months at most before it becomes useless (unless you get the jacket professionally cleaned and treated regularly, which, let's face it, very few people do). In fairness, though, this criticism is true of many DWR-treated fabrics, not just those used by TNF.

If you take a CLOSE look at the photograph of the jacket, you can see that the sides of the hood do not draw in towards the chin - instead, they are cut so as to go straight down to the collar/shoulders of the jacket. Unfortunately, I did not notice this when I bought mine in the UK. The result of this horrendous design flaw is that the jacket is reasonably warm on a still day, but as soon as a breeze of as little as 10-15mph gets up, it rushes straight into the hood, past your cheeks, and down through the collar, chilling your entire body to the core and flushing out the warm body heat through the bottom of the jacket. It beggars belief that this jacket was designed with any consultation whatsoever with mountaineers!I reported this glaring design flaw to TNF Europe and instead of seriously listening to my concerns, they simply told me that I was (supposedly) 'the first person to ever mention it as a possible problem', and consequently would 'pass on my concerns' but it was abundantly clear that they thereafter considered my interaction with them redundant and satisfactorily concluded. In short, buyer beware: TNF appear to have something of an 'attitude problem' - instead of properly considering customer feedback, they apparently seek to diminish its credibility and continue to produce products with known flaws. TNF have achieved a huge growth in market share partly because they have focused upon satisfying the demands of the fashion industry, to appeal to a broader cross-section of the general public. Their design teams are therefore unfortunately focused as much on 'form OVER function' as they are on 'form FOLLOWS function'. I had hoped that, on account of this particular jacket being marketed as part of the 'Summit Series', the design team might have biased their focus in favour of 'form FOLLOWS function', but my experience has sadly proved otherwise.

In conclusion, this IS a nice, lightweight, goodlooking jacket which is great for doing the shopping on a winters day, but please don't believe the nonsense marketing hype that it's designed for serious mountaineering use (or on any day when there's more than a slight breeze). If you like it, then by all means buy it (along with a neck gaiter as, quite literally, a 'stop-gap' solution) but bear in mind that you will still need to buy a more serious jacket in addition, for those days when you want to go somewhere more demanding than the high street or the shopping mall.

If TNF decide to swallow their pride and modify the design of this hood for Winter 2010, they will at least triple the usability of this jacket so please bear this in mind if you read this review in 2010 or beyond. This jacket has potential for fast, lightweight ascents but ONLY IF the hood design is improved to protect the neck and chin.

For those of you wondering what the fill weight is, I don't know precisely, but I estimate it to be approximately 200 grams. It seems less springy than the down in Rab's Neutrino Endurance jacket although this subjective viewpoint may be to do with the fact that the down is more tightly constrained in the Rab jacket, due to much closer baffling (which, appearance-wise, does make the Rab offering look more Michelin-man-like, but of course form-follows-function is the most important thing, so I'm not criticising this aspect of the Neutrino, just pointing it out as a comparative point in contrast to the more widely-spaced welded baffles of the TNF Prism Optimus).

I've had this jacket for 2 years now and am rapidly approaching the 3rd winter with it up in Summit County, CO. It's super warm. I can wear just a t'shirt under it when I need to walk Molly-dog and it does just fine. For colder days/nights(stumbling around the streets of Breck), wearing a fleece as a mid layer is perfect. I don't ski in it (way to hot). But it is a great piece to pull out of the pack and throw on while waiting for the rest of crew to skin up.