As the main contact point on the bike, making sure you buy the right cycling bib shorts is really important. Many riders swear by a particular brand, but every cyclist is different, so it's really worthwhile taking a moment to decide exactly what type of rider you are and how many miles you intend to cover per week.
Like anything in life, you do get what you pay for, and there is a direct correlation between the amount of miles you will want to cover and the price of the bib short. Generally, if you are riding over 150 miles per week, you should look towards top level cycling bibs. At this price point you are paying for sheer quality of finish, the technical capabilities of the chamois, and for the fit. All of these factors translate into your ability to ride for long periods at a time without discomfort.
For riders looking to get out on the bike 3-4 times a week, then a cycling bib short aimed at a slightly lower mileage would be ideal. You will find there is little sacrifice to finish and comfort, but the pad is more suited to less saddle time. A slightly lower price point also brings with it a slightly thicker lycra construction, whereas generally, a top level bib short is constructed from lighter material.
If you're just getting into cycling and either ride small amounts every day, or just like to ride on the weekend, then you don't need anything more than a well priced pair of entry level bib shorts.