1. What is a plunge saw?
A: It is a circular saw that runs along a portable aluminum track.
2. Why would I bother with this instead of my table saw?
A: The tracks are portable and the assembly packs up pretty small for moving things around. A table saw is better as you can set the cut fence once for many repeat cuts of the same width, but the plunge saw is easier to cut large sheet goods if your table is not very big and you dont own outfeed tables.
3. Why would I bother with this instead of a circular / worm-drive saw?
A: The track guides the saw directly instead of being held sideways like most saw guides - this usually keeps the cut perfectly straight once you line up the fence with a square and clamp it to the work. Also many tracks have a special rubber edge where the saw blade runs right up against, which prevents tear-out on nice sheet goods.
4. What are the downsides beyond what I listed in #2 & #3 above?
A: Track length. Tracks can be super expensive if you intend to cut longer lengths. Set up for each cut does not let you repeat cuts quickly. You usually need to buy special saw blades versus common and cheap circular/table saw blades.
I don't work for the seller, but I do have the DeWalt version. FesTool makes the most expensive and usually best-reviewed of the breed, with the DeWalt coming in a close second in my opinion. I spent about 250 on the saw itself with one blade and a 4 foot track, then another 120 for an 8 foot track. Then another 30 for the special inline track clamps, and another 30 for a cool track attachment that fits my router (love it!). A little over 400.00 and I use it all the time.
All told, I could've bought a cheap table saw and circular saw, but I love my DeWalt. The cuts are very clean, the clamps are powerful, and the router attachment is a whole new world of use for the tracks. The plunge saw concept is very light and portable in my opinion.
My recommendation - unless you can get a longer track, this thing is a waste of money. Underpowered, replacement blades are not cheap if they only use FesTool blades, and I have no ideas on the track quality here which is the main attraction of a plunge saw. The link that you see lower in the discussion seems to be an unavailable Amazon product.
More importantly - it seems they do not make an 8 foot track, but rather two shorter lengths that you connect under the guide rail. I strongly dislike those connectors, as you can easily skew the track at the connection and then lose the straightness/squareness of the assembly.
For those of you that want to dip your toes in the plunge saw waters (that sounds dangerous), you really should start w/ a good table saw and circular saw I'm tryin' to think, but nuthin' happens.
When you encounter disappointing scenarios that those tools do not easily solve, go get the plunge saw - not really an entry-level woodworker "must have" tool here...
The Sawmill Creek discussion is really excellent comparison of the product versus the FesTool.
PS you may see deals for a battery-powered DeWalt saw - avoid those like the plague! :-)