Keppra is most commonly used to treat seizures (epilepsy) but a large 2003 study found that its also effective in treating neuropathic pain - which is presumably your problem given you were on Neurontin. Its not uncommon for a drug that was originally developed to treat one problem to be found effective in treating another. Chronic pain patients often go through several medications before they find that something that does the job for them. Good luck! 

You can read about that study below.

http://www.scienceblog.com/community/older/2003/F/20033529.html

For some basic information on Keppra

http://www.healthcentral.com/chronic-pain/find-drug-25240-25.html