Now that I've seen it, I can say that it was disappointing. Generally, it was an annoying movie. The first half hour had Maggie being portrayed mostly in her elderly, shaky mental state - which is completely irrelevant to the life I expected to see portrayed. I felt the urge to walk out but had a large popcorn to finish. It got better, though there was still too much time spent with Thatcher in her dementia - and those parts of her political career she is best known for were given relatively short shrift.
The screen writer and director deserve low marks. Meryl Streep does a decent job, but then she always does. Best actress? I don't think so but maybe that's what they were shooting for.
The professional reviewer I trust most, James Berardinelli, comes closest to what I thought about it:
... The most disappointing aspect of The Iron Lady is that some of the most memorable hallmarks of Thatcher's time in power are glossed over. The Falklands War is covered at warp speed. An IRA attempt on her life gets a couple of minutes. Her relationship with U.S. President Ronald Reagan is confined to a brief scene of them dancing at an official function. By spending so much time with Thatcher in her dotage, the movie becomes limited in its ability to tell the story of her earlier years in a convincing, compelling fashion. One can get a better sense of her importance to recent history by reading her Wikipedia entry.
The pace is dreadfully slow; on those occasions when the flashback scenes generate some forward momentum, we are inevitably yanked back to the "present" so everything can slow down again. This approach can work when a movie features rich dialogue and impeccable performances, but The Iron Lady has neither. The screenplay is pedestrian and the acting, including that of star Meryl Streep, is unmemorable.
[...]Other reviews - mixed: from lousy to over-rated.
Update: Two more views - two Barbaras: Amiel and Kay.