You have to be careful with these types of shares. Generally, you get what you pay for and, therefore, it is likely that the company is not very good.
Also, the spreads quoted by the market-makers on these shares can be quite nasty (20 - 40%), so the share will have have to go up at least this much for you to break even.
Finally, they will be very illiquid and probably trade sideways for months at a time.
Personally, I would avoid them especially if you are new to trading. If you're not and you really do want to trade them, my advice is to definately not build an entire portfolio around them...