Penny slot payout rates are usually lower than those of higher stake machines, because operators set a lower return to player where stakes are small. Penny slots still exist in large numbers, though they look like any other video slot, with bright themes, many lines and bonus features rather than the single coin slot of old. Whether they are worth it depends on treating them as entertainment; how to win on penny slots has no answer beyond luck and a firm budget.
Progressive jackpot odds are long, often one in several million spins on the largest networked games, and many require the maximum stake to qualify. The biggest jackpot on a slot machine can reach a sum that changes a life, which is exactly why the base game usually returns less than an ordinary slot. Whether progressive jackpots are worth it depends on accepting that cost for a small chance. Free slots with a progressive theme pay no real jackpot of any kind.
Progressive jackpots work by taking a small share of every stake and adding it to a prize that keeps growing until someone wins it. The jackpot meter on the screen shows that running total, and on networked games it rises with stakes from many casinos at once. A mystery progressive jackpot is paid at a random moment before the meter reaches a hidden limit rather than on a symbol combination, so any spin may trigger it.