Effectiveness
Terman (1943, pp. 37, Table 18, 78) provides an expression for the ratio of resistance to alternating current to resistance to direct current for an isolated litz wire. It does not apply to windings with multiple turns. An expression for the resistance ratio in windings is given by Sullivan (1999) at Eqn 2 and Appendix A (page 289).
Litz wire is very effective below 500 kHz; it is rarely used above 2 MHz as it is much less effective there.
Litz wire has a higher impedance per unit cross-section, area but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies.
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