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We present a systematic characterization of Mg1[?]xLixB2 polycrystalline samples with nominal Li content x up to 0.30. We explore the effectiveness of the substitution and investigate its influence on superconducting and normal state properties. The structural analyses by neutron and x-ray diffraction indicate that, despite the lattice parameters remaining unchanged within the experimental accuracy, around 30% of the nominal Li content actually enters the structure. Also the transition temperature is only weakly affected by Li substitution, but its relationship with the residual resistivity and magnetoresistivity data is compatible with a picture where p bands are filled with holes and/or affected by disorder, as predicted by theory. We also measured the magnetic penetration depth l by an inductance technique. Data fits based on the standard Bardeen-Cooper-Schrieffer two-band model yield the zero-temperature limit of both l and superconducting gaps: we find that l(0) increases weakly and quickly saturates with increasing x, whereas Dp(0) and Ds(0) decrease with x. This analysis suggests that lithium substitution induces disorder mainly in the p band. Point contact spectroscopy results on samples obtained from the same batch are in full agreement with magnetic penetration depth data on the undoped sample, but give superconducting gaps almost insensitive to Li substitution, showing at most a small increase in Dp(0) that may be related to an inhomogeneous distribution of Li content. [ABSTRACT FROM AUTHOR]