I have made a backtest using the following data:
UPRO simulated 1929-2019 from HPC 3.82alpha
SSO simulated 1929-2019 from HPC 3.82alpha
SPY w/o dividends 1929-2019 from HPC 3.82alpha
TMF simulated 1986 - 2019 from HedgeFundie
Approach: Hold 100% UPRO when SPY w/o dividends trends above SMA200 (of SPY as benchmark). If trends below SMA200, sell all UPRO and hold 100% in Cash.
Results: I've loaded all data simulated in Portfolio Visualizer to apply the strategy. As Portfolio Visualizer is only able to load data from 1985, I've tricked the visualizer by saying that data from 1929 and from 1950 is also since 1985, but that is just so that I can run the analysis.
3'000 investing monthly
Hold and/or invest 100% in UPRO / Cash; depending on SMA200 of SPY
not inflation adjusted (as no inflation data available for 1929 - 1985 in Portfolio Visualizer)
Please see the following screenshots:
1985 - 2019

1950 - 1982 (tricked)

1929 - 1962 (tricked)

1985 - 2019 (using UPRO when SPY over SMA200, otherwise invest all in TMF)

Conclusion
I was very surprised by these results. Please be aware, that the benchmark used (SPY) is without dividends. So actual results when investing will differ as SPY generated higher dividends from 1929 - 1985 than todays 2-3%.
However, the results are quite overwhelming to me as I've used rather conservative approaches when simulating UPRO and SSO. I've also compared simulated SSO to actual 2x SP500 (UPLIX) (see below screenshot for check):
Simulated SSO vs UPLIX - 1985 - 2019

Verdict
Please critically analze the above results and destroy them. Otherwise I would not see any reason why not to go for at least 50% of my portfolio into that strategy (100% UPRO / 100% CASH).