I honestly haven't noticed much of a change. If anything, almost seemed like it improved. After fully charging my 13" i5 mbp, it showed 12 hrs remaining at idle. I don't think I've ever seen the number over 10 or 11 before then. I didn't get much of a chance to really compare the two because the next day I installed the HDD in the optibay. In my limited use, I haven't once felt like the battery was draining faster. I have the 128GB samsung 830 + 5400 750GB HDD (sleeps after 1 min of inactivity). I didn't bother doing any scientific tests because I needed this setup.
My recommendation, don't worry about battery life. It's not enough to distract from the positives of using an SSD. SSD is quicker, lighter, and you will notice how much quieter your computer is compared to a HDD.
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Wow, I'm glad I don't live in Spain for buying SSD's, although I'd love to live there. I have the Samsung 830 128GB, it's been great so far for the few days I've had it. (I paid $140 for it in the US)