I only got around to playing Heretic recently, and I was genuinely surprised at how well it held up. The game is fun, challenging, and good throughout. Some of the controls are a little obtuse at first glance until you get used to them (and button placement is not optimal), so you might want to try to get some configuration.
Some of the bosses can be a little difficult, but it's not that bad. Now the EXPANSION - that one seemed to crank the difficulty up to extremely high levels, so that might give you some trouble. However, it's only two of the five chapters included in the game, and it's all one package, so it's not a big deal.
A lot of fun, and it's held up a lot better than Doom itself has.