Sixth! GHC 6.10 is way too old, it doesn't have several import features like Data.ByteString.Builder (without this doing output is painful, such as the TSORT problem, which produces tons of output); Data.Vector (A much better choice over Data.Array), Also I think GHC 6.10 isn't doing well with Fusion (though I'm no haskell expert), so it runs much slower compare the recent GHC (7.8.3). Not to mention there lots of incompatibilities between GHC 6.10 & GHC 7.6+, such as Data.Map/Data.Set, the interfaces changed so much that I have to get about 5-10 CEs if I have to use some methods from them.
I wrote a Haskell version of NFACTOR, it takes 16.XX seconds to finish, while C++ version with takes about 1.6 seconds, But on by local machine C++ version is only about 2 times faster (I believe if I choose Unboxed vector instead of Array haskell version could be even the same as C++ version), both with 1MB lines of input data size. Another example is problem FACTCG2, I'm not sure it's even possible with GHC 6.10 (I got many TLEs while same algorithm in C++ can pass easily).
Personally I strongly pose to Haskell than other programming language, but it's really painful to submit a solution for GHC 6.10 and sometimes it's almost impossible. It will be really nice if SPOJ can have Haskell platform 2014.2 installed, though I'm not sure if someone really look into this at all -_-.