Compatible Maintenance Kits for HP 4si

Has anybody had any experience using compatible Maintenance kits for laser printers. As the price difference is quite considerable around ?100 in favor of the compatible.

If anybody has, please can you let me know your opinion! As ?100 is alot of money on a tight budget.

If they are no good then i will have to fork out for the genuine kits.

Thanks in advance for any comments/Advice