From a desktop perspective its definately in the 95-98 windows era though the power of what possible there in linux is light years ahead. Unlocking that power for the average users is not easy or really possible for them without coaching and effort from someone in the know.

Now from a server and infrastructure perspective. The linux platform is definately on the par and in some case well above the stability of your current Microsoft offerings.  Perhaps not AS feature laden but stability wise definately on the par or greater. (I NEVER touch my Linux machines running  the few things we have here but do manage to spend some quality time on our internal microsoft servers). I can have this stability with a single machine running most everything on Linux whereas stability on a windows server platform usually means not asking it to do to much. Limiting the tasks for a piece of server hardware limits is interaction surface between tasks and thus increases that uinits stability and relaibility.

From a security standpoint I do feel that microsoft has made tremendous strides over the past year XPSP2 not withstanding. But the fact remains that as long as Microsoft is on TOP they will be the target of choice for the malcontents and general 'bubble gum on the bottom of your shoe' types out there. This is a simple fact and while security seems to be high on a well administered Linux system I feel that is largely due to this less enticing target status. Regardless of the reason though an admin must monitor each platform and react accordingly. Dealing with security on a Linux platform seems to be less of a chore for whatever reason.

From a development standpoint. Linus development is definately not as polished, Visual Studio is da BOMB. C# is a better java than java, .Net Rocks. Of course there is Mono in the Linux camps ( Mono also works well on Windows folks ) and for webservice work in SOA setups Mono on Linux makes for a killer setup. But the development platform where, for the developer anyways, the Rubber meets the road, Visual studio is the only way to go.