Haven't, but the fees are high from looking at the website for Stash. $5 may sound great, but you are paying $12/year in fees.
Fidelity or Vanguard charge basically nothing. You can probably open an IRA at either with no minimum. For the index funds you should be buying, they will both have good ETFs with no commissions, and the funds themselves have low management expenses. I personally would suggest an IRA or brokerage account with Fidelity and set up a monthly direct deposit from your paycheck directly to it (for at least a bit more than $5). I would buy the Blackrock iShares IVV (an S&P500 index) whenever there is enough in the account to buy more, a few times/year, or whenever there is a decline in the market (like last month).