I would recommend that you check out the reviews Here. You can also read the reviews for each of the 101 spells products (o-level through 9th level) that Rite published since 1001 Spells is simply a compilation of those products.
Endzeitgeist is one of the most highly respected reviewers in the industry (and a nice guy to boot) and he has given all the 101 Spells products 4.5 to 5 star reviews.
Reputation-wise: Rite Publishing is considered one of the better companies regarding balance in their products (up there with Rogue Genius Games, Raging Swan Press and Legendary Games).
Here's my question for you: How long have you been GM'ing? Is this your first shot GMing Pathfinder? If so, you have already got a TON of options for your players, do they really NEED another 1010 spells as potential options for them? Do you want to have to keep track of another book?
I play in a game with only the Core and APG allowed and we have enough to keep us having fun for a while.
I GM a game with all options open (except the summoner) including 3PP material (if I get to read it over first and I get a copy if we're using it). We all have a lot of fun in my game too.
I will tell you in my personal experience I played with a person who used 1001 and it was bad. I don't mean the quality of the book or publishing or anything, but that I felt many of the spells that the player used were completely overpowered. I will add this player is also a munchkin to the max, but there were just far too many spells that seem incredibly overpowered compared to spells that Paizo has published.
This is just my personal experience when playing at the game table. I really didn't like it being used, and eventually we collectively (except the GM) asked the player to stop using the spells from the book.
There are probably a lot of spells that aren't over the top, but if you have players who are anything like this guy I was playing with they will go through it and just pick out all the really good ones and the rest will be ignored.
Extra spells help prepared casters more than spontaneous casters. Divine prepared casters get the biggest boost as they automatically know all spells they are capable of casting. Pathfinder with all the extra books already has a large selection of spells. In my opinion all the basic spells you need are already there. Any extra spells will usually be narrowly tailored to specific circumstances. Those are the types of spells that are the most unbalanced.
The other thing to consider is that many 3rd party spells are designed to give the caster access to the types of spells on other casters lists. As it stand certain classes are better at certain types of spells. This creates some needed variation among the characters. When every caster can do basically the same thing it makes it boring. While it does make since that the fire cleric should have lots of fire spells this ends up giving all clerics fire spells. For clerics you can simple only allow them certain spells, but that does not work well with wizards. Also that is a ton of work to sort through over a thousand spells and try and figure out what your players get access to.
If the play experience is anything like the 3.5 Spell Compendium, then including this book will be lots of fun for the players (the core rules spell selection has some weak spots). Fun it an important consideration, and you should weight it against how increase in power that the spell casters are going to experience, and how much work will be required of you to accomodate it. Allowing it might offset whatever measure of balance your game currenttly has. I encourage you to rewrite the spell lists of NPC spellcasters using the book, especially if they come from a published adventure.
I've used a few spells for 1001 spells but outside of myself I have not seen anyone really use it yet. I'm allowing it for an upcoming campaign so I did my best to dig for good stuff and personally for each spell I thought was broken I could find a Paizo spell at the same level and on the same spell list that was worse. There's things that are as bad as Paizo spells but not more powerful so I expect to either see the same spells cast as i always do or new names on the same shenanigans.
In my games I have the 3.5 Spell Compendium and 1001 Spells available as researchable spells. That way I'm not increasing any lists massively (especially for clerics and druids) but they are available with only a quick look by my to make sure it isn't one of the few that I think is broken. This has worked out pretty well so far.