I can offer the following based on my experience
Personally I avoid breaks from G unless essential. I have had 2 breaks recommended by Onc since Aug 2008 (diagnosis time) due to side effects, once due to severe swelling, then over all exhaustion, not feeling well etc. CML wise I responded very well was PCR-U in less then a year.
When I resumed my second break I resumed at 300 mgs, been doing this none months now getting ready for next check (9 months since dose reduction). I was still undetectable at 3 month and 6 month check. Side effect wise it has made a great difference to me, I still get some side effects, but they are much more manageable and they don't last. For example, After heavy physical work get some muscle pain but it improves in a few days, I still get fatigue, but if I slow down and take it just a little easy I can get back in a day or so. I am very active, work full time, have two children, so I am always busy. I feel like now is probably the way a person my age(48) normally feels. Before I would not improve for at least a week and it was like I was always having some issue. Now I feel like I can manage the situation. It took several months for the dosage reduction to really have a positive impact. It happened gradually then at 4months from that time I all of a sudden started feeling alot better. I still get some tough joint pain, but I wonder if it is not related to CML or Gleevec.
Hope this info is helpful.