Hi aad Dira,
I had a bit of a think overnight before replying. What I see with your question raises others with me-
what do you want to achieve with the filter?
You say you "need to buy a filter"- why?
How big is you scope to say you can't see DSOs from the big smoke?
Filters are tricky things. They can "help", but they don't solve all your problem. Remember, filters will reduce the amount of precious light that your telescope is gathering for you.
The advertisment doesn't convince me that it is nothing more than a glorified colour filter, around the one known as #80A Blue. Maybe with a little added violet. It really will only help you with some Lunar and planetary observations. It can't help you with deep sky objects as it is actually reducing the very spectrum at which nebulae glow at, other than a little blue- but our eyes are not very sensitive to this part of the spectrum, especially at low light levels, so it is of no real help. The ad does mention that this filter cannot take the place of a dedicated nebula filter, but it doesn't mention that you won't see more with it either.
You say that you can't see deep sky objects from where you are. If you describe where you are city wise and site wise, we might be able to give you some tips on how to improve your situation (other than knocking out the power grid,
).
If you are after a filter, have a look at this really good filter link. It will explain how they work, both nebula & colour filters:
http://www.lumicon.com/astronomy-accessories.php?cid=1
I have four five filters myself, two nebula (a general 'light pollution' one & an oxygen III), two colour (#80A & #8), and a polarising set (two polarising filters that you can then use to vary reduce the glare of the moon or planets). I don't use them very much. I mainly use the nebula filters at home, but they are no good for all other objects. And I don't look at the Moon much at all, and very little at the planets, so the other filters don't breathe the night air very much.
What I'm getting at is I mainly just use my own eyes to do the majority of my observing. I'll give you an example. On one occassion at a star party here in Sydney, with poor seeing conditions, we tried some galaxy hunting using my 17.5" scope. My 40 year old eyes could see a really faint smudge of a galaxy where a 17 year old couldn't. That was only due to experience- not filters. See what I mean?
Mental.