In a manner of speaking, what your collegue said could be considered true. I had a recent conversation with my trainer on this subject a couple weeks ago, and what he told me is that research is finding that people's bodies tend to adapt to an exercise routine fairly quickly (within a few weeks). Once your body adapts to the routine, then the exercise becomes less effective than it was when you were first learning the routine. The exercise becomes more muscle memory vs taking more energy/working smaller muscles (like stabilizers) when you first are learning. It's not that the exercise becomes completely ineffective - it's just not as effective as it used to be.
The way around this is to change up your routine. Something like having 2 or 3 different "routines" and mix & match them in different ways is the recommended strategy. Switch up the combinations every 2 or 3 weeks. What this does is prevents your body from getting used to any stable routine, which helps maximize the effectiveness.
This is one reason why people get "bored" with all those exercise videos - you buy one or two and work them for a month, and by the end of the month you no longer want to work them. It's because your body adapted to the fixed routine, and basically your lack of desire is probably your body telling you it's "bored" and needs variety.
As far as taking breaks - it is a good idea to have "rest" periods incorporated to allow for recovery period. How much recovery you need depends on how much and what type of exercises you are doing, and what your current condition is. If you do a really hard strength workout combined with an intense stretch session - your body will probably need a day or so to recover (more if you are not used to such a workout). From my experience, anytime my routine changes or they up the weight, I know I need a little extra recovery time as my body is moving up to the next "level". On those recovery days (an in general) - be sure to drink plenty of water to cleanse your system - especially if you are feeling that post-exercise 24 hour burn/ache set in. That is the lactic acid that builds up in your muscle tissue which causes the pain. Lots of water and light massaging, or light exercise (like a stroll around the neighborhood) - helps alleviate that condition a bit faster.
I think your plan to mix things up is in line with the advice my trainer gave me. If you feel you've gotten into a routine or feeling "bored" with your exercise - just thing of something new to toss into your exercise mix to shake things up again.
Hope some of this information is useful.