Everything a DL4 can do and more.

Incredible delays, reverbs and modulations from the stompbox modeler series but lackluster distortions (some adjustments are required to reduce excesive high end). The looper function is better than the DL4 since you access to it using your feet and you don't need to double tap any button for the reverse function. Stock switches tend to break easily so I got them replaced.

Preferred Settings + Usage:

Ditto X2 made me buy this.

A couple years ago I bought a Blackstar ID 30 and Ditto X2... tried to loop through the ID30 ... epic fail. I try to keep it simple. Needed a good delay & chorus. Hearing about DL 4 decided to look further went to my local store and saw this. I didn't need or want a amp modeler multi effect board, I have the ID 30. Upon learning this was the POD 500 sans amp modeling and it was the DL 4 plus it was a workable solution to use with the Ditto X2.

While I have a low opinion of Line 6 in general, I bought it. What can I say, Delays are excellent and love the idea you can stack multiple delays... Or any effects. Easier than TC Electronic Flashback or Flashback Mini. You can set up multiple delays (effects) which beats bending over and setting and resetting for different songs.

Modulations are great too. Distortions sound a little thin but usable though I would not use them for serious performing or recording but, good enough for looping. Filters ...don't use them but nice to have should you need them. Reverbs are good. The 30 second looper is not bad either.

If I were to use one pedal this would be it. Build seems solid though I have read players have had issues with switches. Power supply is clunky. There are many effects to pull from if you need it most you will never use unless your name is Zappa. Foot print is about the same size as the DL 4 so for $100 more why not.

relatively cheap with plenty of timeless sounds

does a bit less than the amount of buttons might initially suggest. but it's packed with all the 4 series algorithms and more. delays and filters are killer, with the reverbs being my least favorite. overall, a great catch-all with up to 3 simultaneous effects

Pass on the DL4 and go straight for the M9.

I was actually shopping for a DL4 and was pointed towards the M9. It takes up roughly the same amount of space on your board as the DL4. It does everything the DL4 does, and also the MM4, the DM4, and some other tasty effects. However, stay away from the distortion (yellow) effects. They are thin, tinny, and feel lifeless. All the other models are fantastic, albeit, not analog. I am finding new, but subtle, ways of using this to keep things interesting for the live performance environment. I highly recommend grabbing at least one expression pedal (you can use 2 for added control) to make full use of the M9. Try this thing, you won't be disappointed.

Excellent but still (just) a Line6

The M series are an amazing collection of effects and a very refreshing change from the typical multi fx board where you have to use the built in amp sims and cab sims. It is without a doubt a feature packed unit with very good sounding classic effects and some innovative ones also, however it is obviously guitar oriented since it misses some basic Bass effects. In the end, it is still just a Line6 unit and, although not bad, tracking for pitch shifting effects is nowhere near an Eventide for instance. Another limitation is the MIDI out port that does not have MIDI through implemented and is not programmable. A couple more turnings to the software and Line6 could sell an update that would turn the unit into a MIDI controller but they preferred to abandone the M series development.

Great compact stompbox modeler

I love the M9 as it has a very good sound for a digital unit, a lot of effects in a very compact unit. I used it for modulations mostly, but it has very good effects for Delay, Reverb, Filters and synths, Od/Dist/Fuzz and it's not expensive. Whenever you don't need a lot of effects (you can use 3 of any at the same time, in the order you want) in a light unit, it's very useful.

Suits some situations.

This is a great idea if you need to save space. Ideal for pit work or very tight stages. The effects are great and varied, there are even effects you will get no where outside of vintage pedals, seek wah comes to mind. The inbuilt looper is effective, not perfect but it works well. This system also makes sense for the casual player at a lower price than many other systems it is well worth it if you want lots of good quality pedals in a small space.

Great pedal, but get the JHV3 mod

Stock, this pedal is awesome. It gives you tons of sounds that work well. However, get the audio mod and switch upgrade from JHV3. The stock switches are on the cheap side. The audio upgrade changes out internal components to higher quality ones and makes a huge improvement. 4 star rating for a stock M9, 5 star for a JHV3 M9.

Use it in front of everything.

Surprised by how good this thing sounds, I use it all the time

It's great.

I use it mainly for delays and reverbs. It's a great unit, but you need to spend time tweaking around.