Like just about everything for the outdoors there's a massive range of 'softshell' jackets to choose from - so how do you know which one is best for you - we'll try and help you here....
What is a Softshell Jacket?: To understand the purpose of the softshell jacket is useful to talk about the performance of 'hardshells' focusing on the potential problems with them, problems which boil down to breathability and stiffness. Creating a waterproof fabric is relatively easy but if the fabric doesn't breathe then you'll get just as wet from sweat build up as you would have done from rain if you hadn't worn it. So, any waterproof jacket (or 'hardshell') will normally have some level of breathability which lets out perspiration through the fabric without letting the rain through in the other direction. Breathability normally goes hand in hand with cost and with technical fabrics the performance of these garments is quite good - you stay dry.
However if you are active, even if you are only walking, a level of sweat can be produced that the fabric can't deal with - you get dampness on the inside of the jacket - even with highly expensive highly breathable jackets. The second problem with hardshells is that people don't like wearing them because the fabric is stiff and a bit uncomfortable.
Softshells were developed to address these issues. In the case of breathability it was possible to create a much more breathable fabric which wasn't quite waterproof - this seems logical - a garment that will let water through more easily sounds like it will let air pass through it more easily as well. And when you think about it, a lot of the time you put on your hardshell because its raining a little bit - or its just a shower. Under these conditions you can now put on a softshell garment rather than a hardshell - so you get protection from wind and showers but with a highly breathable jacket which means there's no build up of sweat and you stay dry and comfortable. Generally the fabrics that go into a softshell are indeed 'soft' - so the garment feels great to wear.
How useful is this?: It sounds like the benefits of a softshell are marginal - afterall your waterproof will keep you dry under most conditions and a softshell jacket won't deal with heavy rain or prolonged showers so you'll need to carry your hardshell anyway - but the reality is much different - softshells keep you so much more comfortable and dry under certain conditions that they really are worth considering. I wore a winter softshell for a full day in sleet and snow and was perfectly dry while my companion's hardshell was seriously damp inside.
Similarly for showery conditions the difference in comfort compared to a hardshell is marked - and because of the relatively poor breathability of hardshells you need to keep taking them off and putting them back on again - otherwise you overheat, a softshell allows you to ride out the showers without having to put your jacket on and off - since its really comfortable whether its raining or not.
What types of Softshell are there?: One of the problems with choosing a softshell arises because there's no standard, so you can get a highly breathable but not very water resistant jacket or a virtually waterproof but not as breathable one - both are called softshells. There are also softshells with fleece liners or pile backed inner surfaces to add extra warmth, which are great for winter but too heavy and warm for summer use. A good guide to the performance characteristics of a softshell is the garments weight.
Softshell jackets weighing around 250grams or less are likely to be designed for trail runners with very high breathability and moderate water and wind resistance.
Softshell jackets weighing between 250 and 450 grams are the most useful, blending good wind and water resistance with high levels of breathability.
At the heavier end softshell jackets probably have some form of fleece lining or backing to the inside of the fabric in order to add insulation - these still come in a mixture of highly breathable or highly water resistant format.
What else to look for?: Hoods? ; obviously personal preference, but if you want a softshell principally for shower protection than a hood may be a good idea. Also you may have a choice between a technical hood or a simple hood - technical hoods have adjustment round the scalp so that you can set the hood to move with your head.
Fabrics? ; Some softshells actually use waterproof fabrics but because the seams aren't 'taped' the garment is not actually waterproof. These will have a high water resistance but breathability may be a compromise - you would be buying this for the softness and comfort of wearing the jacket and its high water resistance rather than its breathability. This format would also be suitable for winter use when it would shrug off snow showers.
Other Features? ; other features and pocket arrangements come down to personal taste, but don't forget pockets add weight - so if you don't use them you don't need the extra weight.
Alternatives: There are some fabric emerging that are waterproof like a hardshell but soft and breathable enough to challenge softshell garments. Polartec Neoshell is a good example - promoted as a fully waterproof hardshell the fabric is softer than 'traditional' hardshell and feels much better to wear. Having used a range of hardshells I really like Neoshell because it is more comfortable and, anecdotally at least, it seems to breathe very well. Problem is I wouldn't categorise it as a softshell, it does't quite match the comfort of a true softshell, I'll still be reaching for my softshell when conditions allow it.