From my experience so far, practicing the SIMS was time well spent.
I am exclusively using Becker and when I studied for FAR I did all the SIMS once and just kind of glanced over them in the week leading up to the exam to get a feel for the type of questions I would see. Much like @skolvikes, I hammered MCQ's and was trending in the low 80%, so I thought I would be ok. When I got to the actual exam, I felt really good about how I did on the MCQ's testlets. Then I started the SIMs. 3 of the 8 I know I got 100% right (including the research question) because they related to what I do everyday at my job. So, I actually feel like I got lucky with the SIM problems I got. The other 5 I was totally unsure about any of my answers and I was kicking myself when I left because I knew I had seen all these same problems in the Becker material almost word for word. I said to myself that if I failed, I would fail because I didn't study the SIMs hard enough. I barely passed FAR with a 76.
When I studied for BEC I gave myself an extra 2 days of review before the exam so I could hammer MCQ's just like I did for FAR (trended in the low 80% again,) but also put a lot more effort into working the SIMs. When I sat for the actual exam I was much more comfortable when the SIMs rolled around, and if anything I felt like I did better on the SIMs then the MCQ's because I got a few IT MCQ's that were totally out of left field. I scored an 89 on BEC.
I know some people will say that BEC is inherently easier then FAR just based off of the amount of information we're tested on, but I think I made a mistake not practicing the SIMs for FAR like I did for BEC. And I think my scores reflect that. The SIMs count for 50% of your score just like the MCQ's do, so while I think it's possible to pass without studying the SIMs hardcore, I don't think it's wise to simply glance over them.