As someone who has been playing through all the Witcher games in the last 8 months:
W1 can be considered an early-2000s flawed RPG with a few glimmers of excellence. The combat system is not terrible, it's just quite boring. The character development is not nearly as well done as W2/3, but there are considerable forays into character complexity that one can be appreciative of, and it is worth playing for the story as it ties into the sequels. In other words, having played it at moderate difficulty, I'd recommend that any fan of the series should play it (if you have played W2/3 and found them merely okay, then skip). It does take a while to get into, and it suffers the most of the three games from quest-interference problems, particularly in Act 2. Honestly, I'd suggest putting it on easy so you can safely ignore most of the mechanics (though that does the unfortunate disservice of also ignoring alchemy, which is important to the universe) and going straight through the story. One thing about the Witcher series as a whole is fetch quests are blessedly rare, and all the fetch quests in W1 can either be skipped, or you may end up completely them in regular gameplay - don't go out of your way.
W2: I'm surprised anyone called Witcher 2's gameplay "****" because it is a straight-up improvement in terms of action and excitement from W1. It does have QTEs - love them or hate them - and it basically requires an awful lot of roll-roll-mash-mouse1-roll in the combat (and the way it handles alchemy is disappointing compared to the first and third titles), but I played through both paths and found the characterization, plot complexity, and even sidequesting enjoyable (played on Hard, which is the 3rd of 5 difficulties). In fact, W2 is a game I'd say is well worth any RPG fans' time even if you opt only to play on the lowest difficulty because I think the morality, characterization, and plot lines rival or exceed anything BioWare has put out, period. It's a game where there is no real "good choice" or "bad choice," merely decision results that have different consequences. That in and of itself makes it a must-play, because any departure from straight up -good-or-evil consequences in games is a rare gem, and W2 does it exceptionally well.
W3: I'm still in White Orchard, so just starting out. Mechanically, W3 appears to take most of the best mechanics from the first two games and combine them, resulting in a much more mechanically satisfying experience (dodge vs roll in the combat, for example, is fantastic; alchemy works properly again and doesn't force you into absurd predictions ahead of time; QTEs are gone), and it seems to take up right where W2 left off on characterization and plot development.
-If you've played any Witcher games and enjoy the story and universe, get W1 (especially if free).
-If you haven't played a Witcher game before and are interested in following the story from the beginning (keeping in mind that gameplay evolves immensely over the series), get W1.
-If you've played any Witcher game before and didn't find anything about it compelling, then give W1 a hard pass.