I understand your question to mean: Is Linux a marginal OS? I say no...
However, if you are asking: Is Linux a mainstream OS which can compete with Windows? I say no...
Not because the software is bad, or its underdeveloped...the biggest strength of Linux is also its biggest weakness. Being free and open source, the community cannot afford a lot of things which Microsoft can. You are not going to see Linux advertisements. You are not going to see Linux publicity campaigns, like the one M$ pulled for Vista. It was an absolute piece of crap so they had to sell it like anything. And this may not make a difference to people like us who are informed enough to make a choice.
But what about the others? My girlfriend recently started using PCLOS too after I convinced her about how great it was. She used the Live CD, loved it...and now she is using it with no problems. However, before I told her about it...she hadn't even heard of Linux. Out of curiosity I asked a lot of my friends (some of them are electronics engineers and computer engineers)...many said they never heard of it. Some said "Isn't it that weird hacker OS"? And very few had ever actually installed it to try it out, let alone use it for any length of time.
IMO, the OS is maturing but very few people are adapting it. But progress IS being made. Dell selling Ubuntu loaded PCs is a HUGE step. So is Canonical Software providing free tech support and free Ubuntu CDs. It will take time...but as of now? Linux isn't in the league of Microsoft. Another 5 to 10 years?? Maybe...I mean, there has to be a limit to how much customers will tolerate Microsoft crap.