Even some DFT methods or beyond DFT methods such as MP2 calculations are computationally very expensive today even with clusters. If you compare QMC (quantum monte carlo) calculations with them, are the QMC calculations way too much expensive? Is it worth to invest time in QMC?
Quantum Monte Carlo (QMC) calculations in various forms, for example variational QMC or diffusion QMC have been used to study periodic systems for decades. In most cases, they provide results that are more accurate than DFT, so it is often taken as a reference for solid state calculations. Indeed, the pioneering QMC calculations of the electron gas by Ceperley and Alder (see here) form the basis of common approximations to exchange correlation functionals, so in a sense enable DFT.
So is it worth investing time in QMC? The method is still computationally more expensive than DFT, so the answer depends. If the system/property you are interested in is well-described by DFT (e.g. weak correlations) then there is no point in doing QMC calculations. However, if you need higher accuracy, there is a definite case for using QMC methods. Another point to consider for the longer term is that many QMC methods are trivially parallelizable, so they are capable of making full use of modern computer architectures, which means that they will probably become increasingly pervasive.
The go-to review article of the use of QMC methods to study solids by Matthew Foulkes and co-workers can be found here.
This is a list of codes that implement QMC calculations for solids that are widely used (feel free to add more):