There are a number of club members enjoying exploiting the characteritics of wideangle lenses for landscape (including me!).
The very nature of these lenses mean that they take in a huge amount of sky and foreground with corresponding vast differences in exposure between the two ( especially when shooting in vertical format).
As far as I am concerned graduated grey filters are an essential tool to balance the exposure between the foreground and sky.
Up until a year ago I was using Cokin P sized filters in a Cokin wideangle holder on my Sigma 10-20. The wideangle holder allows one filter to be mounted. Cokin P graduated grey filters are available in hard and soft and vary in strength between one stop, two stop and three stop. I found the three stop to be most useful for the pictures I was producing although I like to produce strong skys. Some of you may prefer to adopt the more subtle "Duncan McEwan" approach of using a 2 stop filter.
Some people consider Cokin filters to produce a purple cast...can't say I've found it a problem though.

My problem with P sized filters was that I was losing pictures where I needed to stack filters where the contrast between foreground and sky was large...typically at sunset. Had I used a standard Cokin filter holder (which has 3 filter slots) vignetteing problems would have been insurmountable.
The only answer was to go for larger filters. The setup I have now is a Cokin Z Pro holder and soft Lee graduated filters. Lee filters are an arm, leg, kidney and lung but I am getting pictures I couldn't have got a year ago. Commonly I am combining a 3 stop and 2 stop filter .

Alternatives to Lee are Cokin Z Pro, Hi Tech and Kood. The Z Pro filters are priced similarly to the Lees, Hi Tech about half the price and Kood seriously cheap. Hi Tech get a good name and I may well consider them when, inevitably, the scratch fairy comes to my Lees. I have had experience of Kood in P sized format and they are good. Only problem with Kood is that they only come in "dark" ( approximately 2 stop) and "light" ( approximately 1 stop).

I would avoid hard graduated filters as these are designed for use with long telephoto lenses when there is a very clearly defined horizon.

I should make it clear that Kood are available in P sized and the larger Lee sized format.

Here Are some handy links.

http://www.ukcamera.com/kood/pages/filtersq.html

http://www.cokin.co.uk/pages/main.htm

http://www.cokin.co.uk/pages/main.htm




Hope this is helpful.

Guy