I bought REITs in 2000 and 2001 sold them in 2003/4. I bought O last year (with the intention of keeping it forever this will change), and couple this last month as prices started to drop. I think most have been overvalued the last few years and are starting to come back to a fair value. I really do think for REIT price should be based a lot on current yield vs T bill risk free yield or possibly cash flow "yield". In general with the turmoil in real estate I would want a fairly large premium. That said I think investors who do their homework can find good ones.
FWIW Moningstar has the fair value of GGP at 41 3 star, KIM at 31, 2 star, and sounded mildly positive about both.