There are a lot of proprietary so called "waterproof breathable" membranes out there and not all are created equal.

I have had the best results with garments utilizing Gore-Tex carrying the "Guaranteed to Keep You Dry" guarantee, which you'll see as a hang tag attached to certified garments. This guarantee applies to garment, not just the Gore Tex membrane used in it and it is a backing of the garments design and overall function by WL Gore. As per the guarantee, WL Gore will repair, replace, or refund the purchase price for any garment carrying the certification that doesn't keep you dry.

In three layer construction, Goretex products are typically only waterproof to 25psi so if you sit on them, kneel on them, or continuously apply pressure to the garment in one area, over time you'll probably get some moisture that is forced through the membrane. Five layer construction like in Simms waders is waterproof to a much higher psi.

These types of membranes of course have a maximum rate at which they can move moisture and in periods of either high activity or a high temperature gradient between the inside of the garment and the outside air temp, you can create more moisture than the membrane can effectively move and you will wet the coat out from within. This is far more noticeable, as many have pointed out, in fabrics that are not breathable. The key here is ventilation and using the garments in appropriate conditions. Though they cut wind well, using waterproof breathable garments on cold and dry conditions is a recipe for condensation/wet out and your better off with synthetic insulation with nylon/micro fiber shells that cut wind and breath much more effectively. On wet days where you are generating a fair amount of heat and perspiration, ventilating your shell is key. Pit zips make this easier and more comfortable when it is pissing sideways.

Another thing to keep in mind for all waterproof breathable membranes is that when dirt/scum clogs the microscopic pores in the membrane they will in effect be propped open and will allow water molecules to pass through, hence the need to keep these garments clean. The use of appropriate cleaning agents (Nikwax Techwash) and DWR rehabbing products (TX Direct, Revivex, etc) has been discussed here before.

For the occasional user and/or light duty use, sure it probably makes the most financial sense to get stuff that is just good enough for that application and save yourself some dough in the process. The more mileage you put on something and the harder you use it in harsh environmental conditions, the more likely you are to appreciate and even need the higher performance of the more expensive products.