You may or may not notice a bit of a change in effect and potency in a generic product. Although the main active drug remains the same, the "carriers" within the tablet may or may not be different depending on the manufacturer of the generic pills.
I take several tablets a day for a heart condition and the first time I got ones from a different manufacturer I queried it with the pharmacist, he told me not to worry because any drugs sold in this country have to have the same effect, they wouldn't be licensed if they didn't.
Now I get them in various different shapes, sizes and colours depending on who makes them, quite often they come from Poland. Bearing in mind this isn't for me to get a hard on, they keep me alive.
Buying from the internet from an unknown source by-passes all these safeguards, the 'carriers' in those could be anything.