The only sets I consider “must buys” are the modular building sets. If I only had $150 to spend on LEGO per year, that’s what I’d do. I buy sets for parts and they are all chock-full of great parts, and offer an incredible value for what you get. But I can understand if someone only wanted to build spaceships, why those sets would not interest them. But for me, those are the only AFOL sets I own, since most of the others don’t seem to be nearly as good of a value. Besides those, I mostly buy whatever I can get on sale. I’m not really big into having a huge minifig collection, so sometimes I buy sets on sale (20-30% off deals at TRU come along often enough), sell the minifigs, and end up with the parts for free or almost free. For example, I recently bought two copies of Woody’s Roundup for $40 each at TRU. I sold the figs for a total of $60, and ended up paying about $24 total for nearly 1000 very nice parts, after you figure in taxes, PayPal & BL fees, etc. In my experience, buying sets & selling the figs will get you more LEGO bang for your buck even than buying Creator sets. (Even though I love the Creator line) Occasionally I find clearance-type stuff (50%+ off), for those I buy several copies, enough so I can sell some, keep some, and end up breaking even.
All that said, I find it important to budget my spending, and make proactive choices about which sets I really don’t want to miss, and which sets I am willing to pass on. Buying discontinued sets is too expensive, and once you start doing it, it takes away from your money to buy current sets, which leads to missing those and then having to buy them second-hand as well. It’s a nasty cycle. Most of us can’t have it all, so it seems better to me to get the best value out of what money we do spend on LEGO.