There's a big debate around whether having clear filters (basically protectors) is worthwhile - for the small cost and (IMO) imperceptible adverse quality impact. I've opted for it and now sport Marumi Lens Protects on all my lenses, but that's for a different reason than them being "filters" as such.
The problem with filters is they can be a bit of a faff to put on and off and, assuming you have different size lenses, you require filters that will fit all of those. This second point can get expensive, take up unnecessary space etc. and certainly doesn't help with the faff factor.
Which is why people who really want to invest in a filter system do just that - they go for a square filter holder that accepts square & rectangular filters and will fit onto all of their different lenses.
The initial cost is perhaps a bit higher, but you end up with a more flexible system and removing the filters for a quick comparison shot takes a fraction of a second - no unscrewing required.
However, that's only of partial interest to you (for the future) since you've already bought a filter or two. The CPL may be useful but it will depend what you're shooting and what lens you're using it on - more info would be helpful. As
@petersmart says, they can cause odd effects especially with wide angle lenses on expanses of blue sky. Theoretically useful for water shots though if that's the look you're after (i.e. killing the reflections).
I haven't personally tried a variable ND filter. I've read about some bad ones, so just stayed away - it may well be there are good ones out there that do a good job (i.e. are consistent and don't give a coloured cast to the image).
I find an ND filter is useful for certain types of shooting outdoors when I want to use a flash but it's too bright (my studio heads don't have HSS). But that's a fairly specific type of use and to be honest a bit of a "fad" thing for me at the moment. I use a big stopper for the waterfall type images as Arran mentions, but again they can feel a bit "fashionable" and we'll fall out of love with that look before too long, I'm sure. My most used are ND grads which even with lightroom are still great for getting it (nearly) right in camera for landscapes. I love them, but I know others who don't use them ever and their shots are far better than mine
So basically YMMV!
