The original Metroid has its flaws - it had them back then and they're even more frustrating now that SM, ZM, etc has come out - but it's still a great game. You just have to approach it with the right attitude - expecting it to deliver the same kind of experience as Super Metroid, for instance, will only lead to disappointment.
What the original Metroid does require is some willingness to work around its faults/nuances and by doing so, you will appreciate all the good stuff that's inside. The first thing I tell everyone who hasn't played the game before is to make a map. Zebes in the original Metroid is confusing, with its look-a-like corridors and dead ends, and not making a map will make backtracking very difficult. You can get through without one but it's a lot harder. Making a map will help you master Zebes and if you can manage to do this, the game really does become very enjoyable.
The great thing is that the core Metroid formula - seeing areas you can't access, finding items and then returning to the aforementioned areas to progress - is really quite strong here even despite being the first game in the series.
FWIW I didn't play the original Metroid until after Super Metroid, Metroid Fusion and all three Prime games. Once I got over the initial shock - its confusing layout, always starting with 30 health and the floaty controls - I was actually surprised at how strong the core game was. The Metroid formula is one of the most satisfying out there and the original Metroid is no different - in some ways it's even more satisfying than other entries in the series due to the sheer complexity the world, even if its puzzles aren't nearly as deep as later entries.
Metroid even has some things Super Metroid could not match - the atmosphere is at its most lonely here, even if its not as oppressive as, say, Tourian at beginning of Super Metroid. Some may blame it on the black backgrounds and chiptune music but I think there's more to it than that. You feel truly helpless, alone and claustrophic at times and the original Metroid accomplishes this better than any other entries in the series.
I'm not going to give the original lee-way for its numerous faults - flaws that were frustrating back then as well - but in no way is it unplayable or even unenjoyable. Approach it with the right mindset and you will discover the fruits of doing so.