I love this instrument and its worked great for me. I would definitely recommend it to a friend.

The YAS-23 is an excellent alto sax for the money. The action is nice a slick and the intonation is very accurate. They're very easy-blowing compare to some other altos I've tried. The tone of a YAS-23 is "bright" to neutral (like most Yamahas) but that's certainly not a crime. In any case, fit the right mouthpiece (e.g. a Selmer S80 'D' facing) and you can "darken" the tone of any sax if you want. So, YAS-23s sound pretty good - so long as you've got good "chops" (in other words, practice hard in improve your embouchure & technique!) and also fit a good-quality mouthpiece such as a Selmer. It's no coincidence that quite a few professional sax-players own YAS-23s as a "doubling" instrument e.g. someone who plays tenor sax 90% of the time but wants an alto for the other 10% of the gigs they play. Alternatively, a professional alto sax player will own a YAS-23 simply because they need a spare alto for when their main instrument (e.g. a Selmer or Yanagisawa) is being overhauled. The reason professionals often have a YAS-23 as their back-up sax is that it gets the job done and offers good bangs-per-buck.

Anyone who rubbishes the YAS-23 alto sax doesn't know what they're talking about. Here's what a true saxophone expert says about them. Now, this guy really does know what he's talking about when it comes to saxophones, unlike the know-it-all teenagers out there who (in reality) don't know much at all. Here's the review of the YAS 21 (which also mentions the YAS-23) which tells it like it is:-

http://www.shwoodwind.co.uk/Reviews/Saxes/Alto/Yamaha_YAS21.htm

One final point to bear in mind. I've been playing the sax since 1978 and I collect them. Here's a list of some of the altos in my collection:- two Conn 6M altos, a 1930s Keilwerth, Selmer Mk 6 (5 digit), Yanagisawa A9932J, a Dolnet M70 and (wait for it!) the humble Yamaha YAS-23. Frankly, I wouldn't hang onto my YAS-23 unless it had something good to offer. True, it's not as nice as my Selmer Mk 6, but it's still fun to blow and offers almost unbeatable value for money. It's no accident that YAS-23s are so popular among saxophonists!

I have played a Yamaha Yas-23 for the first 7 years of my "carreer".
It is a great instruments for beginners, because, in basic terms, it is a decent saxophone for beginners. It is well-built, quite ergonomic, sounds decent and not too expensive compared to other similar instruments.
There are, however, some points to keep in mind. The first are the pads; they tend to wear out quite quickly. This is mainly caused by the fact that the edges of the holes may be a little rough and sharp. So check them from time to time.
The second is a bigger problem; the mouthpiece. The standard mouthpiece that came with it during the 1990's and the early 2000's was absolutely rubbish. If you want to learn play properly, I would strongly advice you to get another mouthpiece (from day 1!!). The best thing to do would be to get a basic mouthpiece from one of the top brands (like Selmer, VanDoren, Meyer etc). Get an all-round, ebony (so not metal) mouthpiece with a moderate tip-opening. A very good option is the Selmer S80 C*, but the other top brands of course have similar mouthpieces. If possible, ask your teacher for advice on this.

This saxophone great for any advancing soxophone player. It does not have any engravings or a high F# key. But it does have a killer tone and build quality. I recommend the Selmer S80 C* mouthpiece because the mouthpiece that comes with the horn isn't the best. I also reccomend getting it tuned to professional standards and it will sound even better. A great horn.

(my alto was a stepping stone to a tenor sax)
Yamaha were the first to develop these sax with computer design systems. and they were very good in their day (still are). Yamaha are second to none in saxophone build quality, some will say that the YAS21, 23 and 25 may not be the most exotic looking horn but they didnt have an exotic price either. So good quality second hand examples are under 350 pounds. and they are still pretty things desipte no engraving etc.

They have a very nice sound (after some practice) I never needed a different mouthpiece to be able to really blast out a score or get to the high notes (with a 2.5 rico royal reed with the standard 4C mouthpiece supplied a sandard, you can blow a fair while)

As a music educator for the past 20 years, I have recommended the YAS-23 to my students as "the" beginner saxophone. The intonation is very good and the instrument is not "clunky" with it's mechanism like other beginner horns. As a professional saxophonist, I have owned the YAS-62 in both Soprano and Alto and currently play on Selmer Serie II's (sop, alto and tenor). If I had to have a student "back-up" horn, the YAS-23 would be my chose. I also recommend the 4C mpc. to my other beginning students until I think they will not drop the mpc. everyday, then I ask them to get Selmer C*'s.