genomics - How is the sequenced genome of a person useful to him in practice, now?

A human genome sequence can uncover large deletions and insertions in the genome and would give the genotypes of both common and private (rare to very rare) small polymorphisms (e.g. SNPs) and SSRs (simple sequence repeats). From this information, one can learn about some curiosity traits (say, sensing asparagus metabolites in the urine, slow or fast twitch muscles, curly vs not curly hair) and, more importantly, about disease risk. Ancestry and family relationships can also be learned as in, for example, a half-sibling relationship.

As many diseases are polygenic with numerous loci each making small contributions to the variance, it becomes very difficult to fully describe the level of increased or decreased risk. In other words, we don't yet know all the players and so assessing their contribution to risk of disease can't be done with complete confidence. To further compound this problem are both epistasis and gene-environment interactions. Epistasis is a gene-gene (gene-gene-gene, etc) interaction, such that genes A and B separately make no contribution to the phenotype (disease risk), but do so in combination - say in ab/ab individuals. A gene-environment interaction can be described as when an allele associates with increased risk only when an environmental factor (e.g., exercise, fat in the diet, sun exposure, oxygen tension (altitude), etc), passes a certain threshold.

Still, the most powerful predictor of disease risk, onset and progression is family history. A genome sequence approaches that history (half your genome is from your mother and half from your father, of course), but remains fairly uninterpretable in terms of complex traits. You may carry a couple variants posing increased risk for heart disease, but what about the other hundred or so loci that also make small contributions to this affliction? Sure, if we knew exactly what those other hundred were and how much they each contribute and if they interact with each other or the environment. An incomplete picture emerges.

With rare diseases and altered genomes (say in cancer compared to normal) great strides are being made to either identify the small number of defective genes (rare diseases) or the commandeered pathways exploited for uncontrolled growth (cancer). In this regard, much can be learned and will have quicker, but still measurable in years, applications to the clinic.