Jeannie, a 'soft shell' and an 'ultra lightweight waterproof jacket' are two different things. Soft shells are made of ripstop-type or stretch fabrics, sometimes backed with a thin fleecy bonded lining. Lightweight waterproofs are the familiar rather crisp-packety goretex (or similar) garments, but with weight saving details such as thinner fabric, no lining, smaller cut, etc.
Soft shells have been around rather longer than the current marketing would have you believe. It's just that they used to be called windproofs. I wear mine (Patagonia Zephur, see review section) more than any other outer garment. It's fine for keeping off the light intermittent drizzle of a British summer, but I haven't been fool enough to test it in persistent heavy rain. I have to say I wouldn't fancy its chances. The main benefit of this type of garment over a full waterproof is vastly superior breathability. This means you can keep it on all day, rain or shine, which saves a lot of fiddle, especially if you're climbing.
There's another type of soft shell, made of thicker stretchy fabric such as Schoeller Dri. I have trousers made in this fabric, and again they're very breathable and fine in light rain, but will eventually saturate if it gets heavy enough. Mind you the large stretch panels in my elderly Patagonia Stretch Triolet jacket never let water through, and I've worn this in some pretty atrocious conditions. So check the manufacturer's claims for stretch fabrics; some may be more waterproof than others, but probably at the expense of breathability.
I bought my first lightweight waterproof last year, a Marmot Precip jacket.
This is every bit as good as a heavier jacket at keeping out water, and a little more breathable too, though nothing like as good as a windproof. It packs down to almost nothing, so its great for having in the bottom of your pack for emergencies. It's somewhat shorter than a regular jacket, and has a rather crap floppy cutaway hood, but otherwise keeps the rain off as well as a conventional jacket. It has one appalling design fault, which they may have rectified: the two velcro tabs for retaining the folded-away hood are on the same side of the hood, so you can't actually use them (you need one on the inside and one on the outside). Like any Marmot gear, you can buy this about 1/3 cheaper from the US. I only bought this model because it was half price at the time, but having looked round since, I think it has better features and cut than its competitors.
Rock and Run have a good offer on at the moment on a lightweight Patagonia waterproof:
http://www.rockrun.com/shop/prod.html?d=5&t=250&p=1769&sid=ffe17d4f0173db6900783cd9c69346cb#details
