I'm still making above 1% monthly profit (also in BTC), which gives a margin of at least 12% yearly, much superior to any investments offered by local banks and fiat deals in general.
Unless your fiat are Venezuelan Bolivares or Argentinian Pesos, I assume your local currency does not perform as bad as FUN.
Now, talking only about the benefits of becoming premium, consider this: buying the 500k FUN required for max level currently costs 0.07 BTC. That BTC at the base 4.69% interest rate would have produced 0.003283 BTC (~124.1 USD at current rates) in a year.
In exchange, you get 16*365=5840 WOF spins + 1.1725% more of annual interest per year. Let us simplify WOF prize calculation and think that you have a 1/3 chance of getting either 50 lottery tickets, 50 RP or 50 satoshis (we are also assuming that RP value is at its maximum and the roulette is giving 50 units of prizes, we are being as optimistic as we can while staying reasonably realistic). So, out of the 5840 you are expected to get each prize ~1947 times. RPs are technically worth 1 satoshi each, so in satoshis you are expected to get: 1947*50*2 = 194700, or 0.001947 BTC.
So, subtracting what you get from the WOF spins from the interest you won't be receiving, we have: 0.003283 - 0.001947 = 0.001336 BTC (~50 USD). So, you would need to make up that much BTC per year from the extra 1.1725% you get as a bonus. Doing the math, we get: 0.001336 / 0.011725 = 0.1139445629 BTC.
If you have at least that in your balance, then yes, purchasing the membership is a solid investment. If you do not, then you are losing money, at least in the short term. And, we have not taken into account how crappy FUN token is and the crash it has had... neither if you bought the tokens/membership when FUN was more expensive.
Sure, you may win a lottery prize or hit a bigger prize at times, but given the low odds, that is not likely to have a significant impact. I also assumed you get the 16 spins and the full bonus from day 0, which is not the case (but after the second year you get the full benefits, so it is not that much of a big deal either). As a longer term strategy it may work, but you will have to trust Freebitco not changing the conditions of the program and cross your fingers that nothing bad (or shady) happens to the site.
Talking about changing prices, Freebitco folks should make the WOF bonus prize match current conditions. It costs 4500 RP and per spin you only get 29~30 RP. You would need 150 spins to (in theory) cover the cost of the bonus, which at 5/hour would translate into 30 hours of rolling (which is not even possible considering that bonuses are active for just 24 hours). Even at its top 50 RP/spin, you would still need to roll 18 of the 24 hours just to "break even" (those calculations assume that all WOF spins will convert to either RP or satoshis, which is not true, and you are likely to get about 1/3 useless lottery tickets), that is absurd.