Personally, I switched to the Roth 401(k) option as soon as my employer opened it up. This was based on my specific situation, which includes the following considerations:
1. Significant doubt that my post-retirement income will be in a substantially lower tax bracket. This is due in part to a personal belief/concern that rates at some point are going to have to go up (having lived through the '70s, I see today's rates as low. Yes, I said that.) If you think differently, follow your own thoughts, not mine.
2. I already have a significant sum saved in a traditional 401(k), and in fact my employer's profit-sharing contribution contines to go toward my traditional 401(k). I like the idea of the Roth for a little tax diversication.
3. I like the idea that the Roth option effectively allows me to shelter more, because the $18K limit is the same for both Roth and regular 401(k), but the Roth is with post-tax $ vs. pre-tax, which means it's really more like being able to put $22-25K away to grow tax-free (actually, more for us, since we are in the over-50 $24K realm).* Since we currently bring in substantially more income than we require for our expenses, it is worth it to me to take the up-front tax hit to be able to get the tax-free growth on more "effective" dollars -- I mean, that's almost $50K/yr that will grow without taxes, ever!
4. I don't have to argue with DH about upping our Vanguard contributions. DH thinks we already save enough outside of the 401(k)s and doesn't like it when I increase our monthly Vanguard transfers. By switching to the Roth, I get to save more effective dollars now, all the while avoiding the argument entirely. (Yes, taxes are higher, but he is going to curse those regardless). I even convinced him to switch his 401(k) to a Roth, too. Heh heh heh.
*Yes, I realize that I could do a traditional 401(k), save several grand in taxes, and put that several grand away post-tax into Vanguard, and that I might come out better in the end doing it that way given the low capital gains tax rates I'd pay on the Vanguard funds. No, I have not run the numbers, because this is where points 1, 2, and 4 come in. I have significant traditional 401(k) investments, which are great if my RE tax rate is lower; I have some regular post-tax Vanguard index funds, which are great if capital gains taxes remain lower than regular income tax rates; and now I have the Roth money, in case we go back to the 1970s and tax rates jack up and capital gains are taxed the same as ordinary income. IOW, I see it as more of a defensive or risk management measure, with a side of marital argument avoidance as the cherry on top.
YMMV. Many, many smart people here disagree with my approach.