Sleep learning is largely scientifically bunk because the parts of the brain that transfer information from short-term to long-term memory are inactive during sleep. The only reason you remember anything from a dream/sleeping is when you wake up considerably fast enough so that your short term memory (which can hold about 7 +/- 2 items for as long as you focus on them) is still centered on the dream after you've woken up.
Lucid dreams may differ, but subconscious learning during sleep doesn't make sense. Research has shown that when you wake people up during Stage 1,2 or 3 sleep they are generally thinking fairly normally, however nobody remembers that they were even conscious during this period the next morning. Why? Because long-term memories weren't being encoded during this time.
There may be some technique that can bypass this, but I'd be very skeptical before investing heavily in such a product.