Low rates support high P/E multiples on stocks. 2% S&P 500 dividends plus the probability of short to intermediate term asset growth make it the only game in town. Low rates look like they will continue for quite some time.
The S&P 500 has weathered several (4 ?) moderate downturns in the past 24 months, motivated by a variety of fears (valuation, oil, Europe) and has a solid floor built in. Exuberance is low but fear has abated a bit. Sounds like a setup for lower volatility and steady upward price creep in the short term.
Given the globally entrenched low rate environment, I feel that the S&P is correctly priced for this low-rate regime or just a tad high but nowhere near the bubble-popping stage based just on index price. In fact I think it is poised to move 10% higher in the next 6 months.
One of these years, rates will be higher. At that time, multiples will have contracted significantly and we will all feel poorer in our stock accounts. Play it like we always play it, by picking the asset allocation you like based on risk tolerance and needs and rebalancing every 3 to 6 months, pocketing the stock gains along the way. Keep in mind a moderately ugly scenario (30%-40% correction lasting 1-2 years, say) and make sure that your plans and asset allocation are tuned such that you can live with it. The higher it goes from here forward the larger the eventual correction.
I honestly could see the S&P 500 at 1500 or 2500 in the next 12 months. I feel that 1500 is less likely. I just have no clue and won't bother trying to time it, other than using dips to shovel a little cash into the Roth if I manage to catch it.