Is it worth getting an OIII filter?

Yes it is! I ordered the Baader Planetarium Oxygen III filter from agenaastro.com. I have now spent several nights observing planetary and emission nebulae and the view through this filter is hands down amazing. Looking at the Veil Nebula is very difficult because it is so faint, but the OIII enhances and brings out every detail as well as seeming to brighten it. I was able to view both parts of the Veil and actually follow the entire ribbon from one end to the other making out every little wisp. The next Nebula I went to was M57 the Ring Nebula which normally looks good but with the filter I could make out more detail on the outer edges which I could not make out with the filter. The Swan Nebula showed so much more detail and textures in the cloud structure. Looking at the Lagoon Nebula was the same showing greatly increased detail but the filter icreased the size of the nebula by 40% by bringing out clouding that you cannot see without the OIII. Now the best view of the nigh was the Orion Nebula. There is so much there to see and the OIII adds so much contrast making the background velvety black that it seems to increase the brightness of the nebula. From what I have read and discussed and seen the OIII will work well in any scope with 8" of aperature or more. This one is hard to beat. This filter is among the narrowest transmission O-III filters on the market as such it effectively blocks the entire visual spectrum with the exception of the O-III transmission lines. Since the transmission of the light in the OIII is so efficient, fainter nebula seem to appear brighter and the extent of nebulosity that can be seen is greater so in fact you will see more of the fainter components.