There are two main benefits: Speed and lower failure rate.
The main drawback is that SSDs aren't necessarily as robust as HDDs over the long term. Eventually they will fail into a read-only mode, but while they work they tend to be much faster, and there are no mechanical parts to fail.
I repair Apple systems for a living, and it is pretty rare to find a bad HDD. I probably see more than most people, but out of probably a couple hundred systems I've fixed, I'd estimate no more than 10% involved the HDD.
My two cents is that SSDs are something to keep an eye on, but not necessarily quite there yet for most people. They have been making some impressive strides however. They've really only been available mass market for a couple of years, and they've already nearly caught up to platter based HDDs in price, capacity, and longevity. In another 2-3 years, it's entirely possible they will overtake platter based drives. So, maybe by the next time you buy a system things will be different. And of course it's a relatively simple measure to replace your HDD with a SSD later. You will forfeit your warranty coverage on that particular component, and POSSIBLY the entire system, though MOST Apple repair shops will still honor any warranty so long as the issue isn't traced back to the SSD you installed. Though they can use it as justification to refuse service. We actually had to do that after a customer kept insisting that there was something wrong with his system, when everything pointed to the fact that it was the HDD he installed. After 2-3 rounds of this, and my providing a pretty detailed analysis showing how every other component checks out, we had to pull out the Apple warranty agreement which does say that it doesn't cover any unauthorized repairs or modifications made to the system. It's rarely enforced, but it's there to deal with problem customers who can't take a hint, and so you should at least be aware of it.