I do work for quite a few mystery shopping companies (and may get round to reviewing the others ... watch this space!) but Grass Roots was the first one I signed up with, and seems to be one of the best.

To register you complete a simple form including a paragraph on what you think makes a good mystery shopper. Good observational skills, attention to detail, a good memory, and the ability to write clearly and concisely are all important.

Mystery shopping is a way for companies to get a real idea of how they are doing on customer service, as you act like a real shopper at all times. You are given a scenario to follow - e.g. an enquiry about Pay As You Go phones - and you have to keep a (mental!) note of such things as who served you (name and description), what they did or didn't mention, how busy the store was etc. Then within 24 hours you enter your results online. You see the questionnaire in advance so you know what questions you will be asked, and therefore what you particularly need to watch out for.

Your comments may result in someone getting a commendation or a reprimand so it is important to be fair. If, like me, you think customer service is important, then this is a good way to help companies improve.

I find it enjoyable to do - it doesn't earn a fortune (typically £10 per assignment) but if you register with a few companies you can sometimes stack a few up and have a mystery shopping day! It's flexible too - you can register up to five postcodes and can accept whichever assignments you choose. It's not a case of rejecting any - you just accept the ones you are interested in. I work part-time so sometimes do a shop in my lunch hour near where I work. Other assignments I accept for towns near where I live.

The downside is you need to register as self-employed, even if you are also employed - though this is simple to do. One point to consider depending on your circumstances is you can apply to be exempt from paying National Insurance if your projected earnings are low enough - and you'd have to be a pretty dedicated shopper for them not to be!
So far I've earned almost £250 in 7 months - that's doing 20 assignments in total. Not megabucks, but certainly reasonably substantial 'pocket money', even when you take into account the taxman's chunk.