You can visit Cafepharma.com and read on the Glaxo board
several threads about this... from the sales reps point of view.

They hate it. They say doctors won't write for it. etc etc.
There are several threads there if you search "Horizant".

What the end of the article hints at is that the sedative effects are substantial. Hence FDA would like to see a SMALLER dosage form.

Gabapentin is not a great pain reliever anyway. Having a long acting form, would just extend the sedative aspects perhaps. It probably knocks those patients with RLS out, so they are basically unconscious! And note... this is a drug originally developed for epilepsy, and not one paper for the new form on this topic! Very strange. One would think a long acting product would be more stable for seizure patients. Duh?
I did a PubMed search and only one page came up ---6 entries.
And that is a very small set of published papers. I suspect the negative ones were not published at all! And the studies were very small compared to other drug studies.

One thread over at Cafepharma suggests that the RLS approval was just to get it on the market so that doctors could be persuaded to write for it "off label". In addition no insurance will pay for this drug off label..either. So it will be expensive!

I just don't see much happening with it. Appears to me to be a Big Pharma manipulation... is all.

Lyrica goes generic soon too! Unless some smoke and mirrors in courts, slated to go off patent Oct 2013.