Usually, I book first class when I know exactly when I'll be travelling (taking advantage of the advance rates and my BahnCard First 25, this is possible in ~90% of my train travel) and I use upgrade vouchers when I need to be flexible (buying a second class full-fare ticket on my BahnCard 50).

I'm able to get about 6 one-way upgrade vouchers per year, so I actually pay only slightly more than what I'd have to spend if I travelled second class using a BahnCard 50.

There were times when consequently travelling in first class using a BC50 for second class and a BC25 for first class was even a bit cheaper than using a BC50 all the time to get 2nd class tickets - but this, unfortunately, is no longer the case with the abolition of the 1.5x first class award points ratio. Still, I'm enjoying the benefits of first class train travel, especially when I'm spending several hours per month on trains.