It happened to me to find micro sd cards not respecting class written on them. there are a lot of softwares that mesure writing and reading speed. Personally, I would spend a little more money for a good product i.e. I have a 32Gb Samsung, Class 10 and I payed it 35€ (shipping included..)
If you don't need a big capability, a good 8gb or 16gb can cost you not more than 10-15€...
It's something you buy once and least for year..buy it good and you won't regret it!