I've been holding it for close to 10 years, it's my best performing holding by a long shot. I like to hold leveraged ETFs in my tax-free investment account and TQQQ along with UPRO are the ETFs of choice for me.
No, the reason I hold them in my tax-free investment account is because you can't trade on margin in those accounts. So the closest thing to margin you can get is a leveraged ETF. I never have to worry about getting margin called, or going into debt with a leveraged ETF, those things are not possible. I like to think of 3x leveraged ETFs as letting me take my tax free investment account and tripling it.
I can certainly lose a lot of money, the fees are substantially higher than a regular ETF (about 4x higher), and the volatility and constant rebalancing on a daily basis results in a phenomenon known as volatility drag... and yet TQQQ and UPRO have been an absolute killer over the past 10 years.
In my non-tax free accounts I hold unleveraged ETFs: SPY and QQQ.