I'll admit that I am a purist. All of my track, save for just a few pieces, are genuine BRIO. If you buy a BRIO set, then I've of the opinion that you should buy BRIO track to expand it. The same goes for the other wooden railway sets: buy Thomas track for Thomas sets, Maxim track for Maxim sets, and so on. But as I said, I am a purist, and most people (and children) don't care which track they use as long as it works.
Unfortunately, that "as long as it works" part is the rub. There is no established standard for wooden track gauges, and while just about everyone uses the BRIO system as their guide, to avoid patent issues each has small deviations in their measurements. These small deviations can lead to derailing when tracks from multiple manufacturers are mixed, particularly in tight turns. There are even variations in the male and female connector sizes, making for tighter or looser fits at joints. Some 3rd party track does not even fit well with other 3rd party track, so mixing them can lead to track that does not join together at all.
In addition to the variations in track cross section, there are no set standards for track length. Again, most manufacturers follow the BRIO system, but U.S. companies may use English rather than metric units, leading to slight variations in length and curve radii or rounding to the nearest quarter, half, or whole inch. This means a BRIO and non-BRIO track may not be exactly the same length, leading to asymmetric layouts.
Further complicating matters is the fact that some third-party manufacturers are not exact on their advertised lengths. The 3" straights advertised by Maxim, for example, are actually 82mm long, which is just over 3.2".
Last, there is the matter of quality. BRIO track has a smooth finish with no rough edges, even along the grooves, and is manufactured from beech wood. Third party track is generally made from a hard wood for durability, but not all manufacturers state the wood variety or even the type. Even if it is made from a hardwood, quality can still be a concern: rough edges are not uncomon and they can even lead to derailments.
Reviews of selected BRIO-compatible track are linked in the sidebar at the right.
That being said, there are still several legitimate reasons why someone may want, or need, to purchase non-BRIO track:
This is not as true as it once was. BRIO track has dropped in price significantly, in particular now that manufacturing has been moved to China where most of the 3rd party track is produced. BRIO has also moved away from selling track as individual packs and instead encourages the more cost-effective expansion packs, though this does mean that you'll get track you may not want or need along with the track that you do.
This is true for a lot of track pieces, and it's a very valid argument. If you want, for example, one of the retired switches F, G, F1, G1, I, J, O, or P, you have two choices: wait for them to show up on eBay at a reasonable price, or buy third-party versions.
Also a very valid argument, particularly when trying to solve track gap problems or create highly customized layouts.
Two of the third party tracks that I recommend every BRIO set have are the 3" straight track from Maxim, currently available from WoodenTracks.com and Amazon, and the 3" straight track from Jesse's Toy Box, currently available from Amazon. The Maxim track measures 82mm long, or about 3.2", which is very close to the A3 track that BRIO did not widely release. The Jesse's Toy Box track measures at 74mm, which is even closer to the A3 track.
These are good tracks for solving gap issues, particularly those that are longer than A1 but shorter than A.
If you don't care, then you don't care, and that's fine.
If you do mix and match third party track with your BRIO set, try to keep track of which is which. If you ever go to sell your BRIO set, whether it be eBay, craigslist, or some other method, it is always a plus to be able to identify which track is BRIO and which is not. People do pay more for genuine BRIO track.
There is no established standard for wooden track gauges. The image, courtesy of Wikipedia, shows the cross sections of various manufacturers' wooden railway track.
Disclaimer: These reviews are based on track that I have personally purchased and used. No one sends me track to review, nor am I compensated in any way.