From an international investor's perspective I think RE in the U.S. is still attractive (and cheap) in many markets. There is still an incredible amount of good fundamentals, transparency and data available when compared to the other RE markets of the world... and prices in the big cities of many countries worldwide are usually more expensive and even yield less. Not to mention that you are exposed to their local currencies which recently have not done too well when compared to the US dollar. The way I see it is that overall I think things are OK, but looking deeper every state is like a different country... and even within each state, different counties, cities and towns are all different, so it really depends.
If there is a big worldwide economic crash in 2015, investing in real estate might even be a good way to protect your wealth.
I think if you are quite conservative and do not leverage up too much, look for a good yield and focus on markets that are diversified and not only reliant on a few industries, then I believe everything will be OK.
So yes, I do see some RE markets in the U.S. having bubbles while most do not. It is possible that those few markets with bubbles will pop in 2015 especially if oil prices continue to stay low throughout the year. In 2015 I will definitely be cautious while investing but I plan to still be active and hope to find some good deals.
Wishing everyone a happy new year and all the best for 2015!