Agree with most posts on here, it's a great piece of kit but does have it's issues and limitations like already mentioned.
It works best on thicker books (28 page and up), if it's just say a 80gsm 16 page book it usually won't flatten and trimming won't make much difference.
We got ours second hand and have had great use out of it (do a few basic monthly newsletters plus other infrequent book jobs and the machine is self maintained so no click costs) but we keep our kit longer than most, would suggest that if it's at xerox price and you change your machines every few years you will struggle for a return on investment, if it's a cheaper 2nd hand one not from xerox then then it could be worth a punt to see how you get on with it but would guess that xerox won't repair it under contract (although ours is still going strong without breaking).
If it's a choice of the squarefold trimmer or an entry level offline booklet maker then it's a whole new question..............
I would suggest the type of work that you do / aim to do would in my opinion be the deciding factor depending on what other kit you have available to use, if you are doing basic paper stock that doesn't bleed and is predominantly a3 fold and staple to a4 colour and/or black and white jobs that have quite a few sheets to them then a squarefold trimmer would be a great asset.
If you are doing a lot of smaller (in terms of physical size) booklet jobs, full bleed booklets, custom size booklets, thicker or laminated cover jobs then an offline booklet maker would be the way to go.
A few points to consider:
Assuming you have the xerox on contract and are paying per click regardless of size running a4 fold to a5 books will cost you twice the amount in click charges running booklets inline rather than using a guillotine and offline booklet maker. Greyscale clicks won't make much difference but colour clicks soon add up especially on longer runs, if you are selling print then you can't be competitive on colour a5 booklet jobs.
Inline won't allow full bleed books (cover yes by pre-printing and using the inserter) - although we do fudge it around sometimes to finish complete books from the inserter tray (print using sra3, cut down and then print from the inserter tray). If the bleed is on the trim edge and high coverage static usually drags the strips through triggering sensors thus creating jams.
The limitation (at least on the 700 machine) for the cover is 160gsm, although have managed 200gsm creased covers but it really rattles through the finisher (doesn't sound good).
Bespoke size booklets are a pain inline and also very limited.
