SMS spying is now a reality. With just a credit card, people can now download the software necessary to read the SMS texts that a phone both sends and receives, this same software can also usually track calls made, received and sometimes even pictures taken, the location at which they were taken and track, through gps, where the phone has been. There are some cheaper variants of such software which might just be able to spy on the SMS texts but for the price of the complete package you might as well go the whole way.
In theory SMS spying software is not much use for actual spying as the installation cannot be done indiscriminately or remotely, however, once it is done it will run in the background and not bother the user at all; they will not know its running. In practice, if you want to spy on someone’s mobile phone usage then it’s more than likely you will be able to get some free time with their handset to run the install if you give it some thought.

The most moral and acceptable use for mobile phone spying software is probably to keep tabs on your children, this could be done either with or without their knowledge and could be done in one of two ways:
You could keep a close eye on the messages they send, the calls they make and the pictures they take or, if you are more trusting, bear in mind if you child has reached 16 then there could be legal implications.
Just leave the software installed and as and when you worry about where they are, when they are late or anything of that nature you could use the software to spy on their location and put your mind at ease.
Knowing that the latter of the two above options is available to you as and when you need it is often enough to justify the cost of such software in itself.
The way people communicate is has changed and is still changing, young people prefer to send text messages to one another, its quick, easy and discrete. This means that a text can often be used at a time when they phone owner is under adult supervision and would not get away with making a call (for example in class or other setting). In addition, with Smartphone development sending a SMS can be so discrete that it may just look as though the child or teenager is playing a video game.
This means SMS spying is the perfect way to gain an insight into what your kids are doing when they are out, it can also be spun as a positive as it can stop you from asking your child to call you every so often when they are out to let you know they are okay, however, this is not likely to be enough to make them like the idea. If they know about what you are doing then they could well end up using a second phone which is definitely something you want to avoid.
Of course if you are thinking of using some spyware on a phone belonging to someone who is legally classified as an adult you shoud definitly think about taking legal advice first, also consider that there is a fair bit of “phone spying softwarre” out there that does not work at all but you will still need to pay for it.