Hi all! My first post on this forum - thanks to Canon Bob for telling me about it.
Most SLR users fall into one of three camps.
1) Never use protective filters
2) Only use them in adverse conditions
3) Always use a filter.
I personally always use a filter - I like my lenses clean all the time, but hate risking damaging the coating on the real glass so would prefer to wipe a filter clean than the real lens. (I think in reality coatings are quite tough these days though).
Quality - I now always use the Hoya Pro filters - they are clearly better when examined - you don't get the internal reflections of cheap filters - but I'm not convinced this makes a massive difference to the resulting photos.
Effect on Image Quality - I'm not convinced there is an effect most of the time, although I accept shooting into the light, filters will increase the prevalence of flare. Also some lenses (e.g. the Canon 100-400L) have a reputation for reacting very badly to cheap filters. People have been convinced they had a faulty 100-400 as they couldn't get a sharp shot, they ditched the filter & bingo. I don't speak from personal experience on this point.
Other issues - some feel filters, far from protecting a lens, instead put it in harms way - a dropped lens with a filter may shatter the filter sending shards to the lens below - I personally think that this is an irrelevant argument as each impact & drop is different so it depends on how the accident happened.
Overall - I like clean lenses, I don't like cleaning lens elements and I get sharp clear photos from all my lenses & they all have UV or Protector filters on them.
Phil