There is benefit to learning both approaches.
Apart from the historical value in the xib approach, xib's also provide modularity. Perhaps you have a library of code or wish to share a useful widget you made. Taking the xib approach would facilitate that sharing and reuse.
The xib approach also allows you some greater flexibility in terms of your own code. For example, iOS 5 contained a bug with UITableView and Accessibility/VoiceOver support that would cause -dequeueReusableCellWithIdentifier: to return nil despite being documented otherwise (see this blog post for further details). To dynamically load table view cells from xib provided the ability to work around the bug.
While the table and tablecell support in Storyboards is wonderful and provides support for what most people need to do in a table, sometimes you have to color outside the lines, you might need lots of different cells, and again, dynamically loading from xibs can be your solution.
One big advantage of Storyboard is the ability to view your entire application's GUI flow. Zoom out and you can see how everything interconnects and flows. With xibs, while the modularity is nice, it's tougher to envision how everything connects and flows together. This can be a useful feature for yourself, or if you have a larger team to share with, to allow others to see how the app flows.
There's value in both approaches, and it's good to know both so you can pick the best tool for your task at hand.
Update 2014-10-06 - Since I wrote the above, I've been involved in more projects. Some with xib, some that could use storyboards.
Storyboards have matured a great deal (we're at Xcode 6 now), and there's a great deal with them that's so nice. I really love how so much more can be done within a storyboard that is a bit more complicated in a xib-approach. A couple examples:
One is when working with UITableView or UICollectionView how much you can work with prototype cells directly in the storyboard. A lot of nice and easy setup, most of the heavy lifting can be in the storyboard, less code. It's quite nice. Trying to do this in the xib approach is certainly do-able, but there's a lot more work to make it happen.
Another is how nicely you can transition between UIViewControllers with the regular segues then go back with unwind segues. All right there in the storyboard, with minimal code. It's just so handy.
But the one thing that still kills storyboards for me is trying to use them in collaborative environments. It's just not going to merge well. And in some regard, it's not even if you're working on a team of > 1 person. If you yourself take advantage of version control, use a good branching and merging model for your own personal workflow, there may come a time where some change is going to have to be made in some branch that has to be brought into another branch, and oh the pain. To me, this is what kills storyboards.
As time and work has evolved, what I'm finding for myself is storyboards are great for prototyping. The ability to get things going quickly is a huge benefit of storyboards. There's much speed in using them. But the speed comes at cost. When it comes to writing the "real" code for some project, I'm just going to stick with xibs because while it may be more work, it's a more flexible route that just works better in larger teams or over time.
Update 2015-04-07 Another update, because the projects of the past some months have forced me to use storyboards, which has provided more insights.
First, some things will mandate one approach or another. For example, apparently there were some edge case bugs when working with size classes in xibs that didn't exist doing the same thing in a storyboard. So if you get affected by bugs, that may force your hand one way or the other. Another is to remember that storyboards generally work on the UIViewController level, so if you need to do something like have a UIView or a UICollectionViewCell to load, that's probably going to be better served by a xib.
Second, and I don't know why this didn't occur to me at first, but there is nothing that requires you to use a singular storyboard for your entire project! I think the nature of storyboard enables people to gravitate that way, but we have to remember nothing mandates that (that I'm aware of).
What I've found works well is to generally approach each "view grouping" per storyboard. That is, often your ViewControllers tend to be isolated and wind up being 1 per storyboard (or xib). But you might have a situation where you have two closely-related ViewControllers, and it makes sense to put them into the same storyboard, especially because then you can easily hook things up between them, such as segues.
The main advantage to multiple storyboards? Working in teams. This way Fred can work on his storyboard and Wilma can work on her storyboard, and there's no strong worries of merge problems or work coordinatinon! The use of multiple storyboards (and generally 1 ViewController per storyboard) has been a huge help in the use of storyboards on a multi-person dev team.
It's pretty evident Apple wants us to prefer storyboards, and I'm embracing them more these days. Using multiple storyboards, but still using a xib when needed, is working fairly well now.
Update 2015-09-21 Now that Apple's released Xcode 7, there's even more reason to adopt storyboards, as Apple works to overcome the shortcomings.
The most important improvement is storyboard references, which allow you to create in one storyboard a reference to another storyboard. It's dead simple to make, and now you can have cross-storyboard segues (both entrance and exit). I've used this a few times already on a new project and it's just a joy.
Another improvement is that you can create stand-alone UIView classes within a storyboard. However, as of this writing I've had mixed results with it. Simple cases work out ok, but some more "complicated" stuff did not. For example, I had a UIViewController with a UITableView within it. Since it was to be a simple table with 5 static cells, I merely instantiated the 5 UITableViewCells as a part of the ViewController in the storyboard. Seemed to work, but then at runtime nothing would actually load and show up; moved the UITableViewCells into a xib, and all worked. I'm not sure if I was doing something wrong or what it may be, so YMMV. But still, even if there's just some quirks, in time I'm sure Apple will resolve them and then another barrier against storyboards will fall. I would say that if you need such support, you should try it and see how it goes for you. There's great promise.
More and more, storyboards are shaping up to be excellent.
