opinions on Marshall MG-DFX amps

To be honest they are rubbish compared with similar priced amps. I started out on a DFX30 amp and it was ok at best. Have a Vox AD30vt amp at the mo and that is much better overall, additionally the Roland Cube amps are nice.

Check those out instead in my opinion.

I've got an MG30DFX, propped up on one of these http://www.musiciansfriend.com/product/Guitar/Amps?sku=450811 . (As much so I don't have to lean over to see the controls as for better projection. :? ) The amp was an eBay deal I got last summer, and I bought the P801 footswitch from Musician's Friend after I had the amp awhile. It was the first amp I owned with that much wattage and that big a speaker.
The reverb, chorus, and other effects in it are okay, although you can only use one at a time. I still use pedals in addition to/instead of the built-in DFX stuff.
I could probably tweak the gain and other controls to get some better OD sounds, but I'm not impressed with the Overdrive channel on it.
The MG30 got an 8.55 overall (from 244 reviews) on the Musician's Friend site. The comments there are for about half of those. One guy who panned it (no rating above 2) suggested a Crate VC-508 tube amp (which I've seen suggested mods for on GuitarNutsâ„¢) instead. Others gave it all 10's.
If you can try one out, preferably side-by-side with any of the others that anyone here recommends, or any other models that interest you, that's the best way to decide.

I owned a Marshall MG50RCD about 5 years ago. This was just before the models with effects came out.

I really liked it, it was the best sounding solid state amp I ever owned. It had a very good (not great) clean channel, and I also liked the overdrive channel. It sure sounded Marshall to me.

The construction of this amp was not the greatest, I had to tighten all the screws all over the amp about once a month. Also, the footswitch went bad, and the gain control got stuck on full gain regardless of where you turned the dial. I was able to fix this though.

It had plenty of power and I used to gig regular with it.

My 2 cents.

I have a MG15DFX and overall I like the amp, and a lot of the features that it has, that said... I have played on better solid state amps since I got this one.

Let me try and give a balanced view of my opinions of it. So I'll list good and bad! First my style of playing, I play Jazz, fusion, and Blues mainly with an occasional country style accompaniment. So I look for a very specific sound out of my amp.

Good
A very good sounding Clean channel that is very responsive to minute input changes and makes my 335 clone sound very nice through it.
3 EQ controls that are responsive and sound great.
Reverb, Chorus, Flanger and Delay that all sound very nice and can be modulated to a little or a lot.
Price was a can't be beat for me. I got a very good deal from Musicians Friend on it.
Inputs and output. It has the capablitiy to allow input from a CD Player or any standard Jack. That is a very good thing when you want to play behind some backing tracks and such. It also has a decent headphone/Line out jack that allows me to practice without waking the family

Bad
The OD channel. I do not like the sound out of the overdrive.When I play with distortion, I'm playing Blues and the OD channel on my amp gave me a very dry stale sound that had no warmth to it. It was either too harsh or not there. Using the Semi-hollowbody just didn't sound right at all through that channel. On the other hand my nephew plugged his strat clone into it and liked the OD, but he's more a punk/metal player.
The effects are one at at time as DougC mentioned already.

Conclusion
Would I buy it again.... In a word... No.
The price I paid for it was a very good deal, and if I could get it at that price again, I might, but it was a one time special, and in buying it as a closeout gig set I got it at a much lower price than you could buy the amp by itself. If I had to pay the current price for that amp I would not purchase it if I could find a tube in the same price range, or even one of the better modeling Solid State Amps. The sound and style I want/play was achieved by adding a Blues pedal to the Clean side and using the Reverb. If I wanted to stack the other effects I'd have to have pedals for them. The other features I wanted/IE: the CD Jack I've used 1 time! :) so what does that tell you about my need for that one!

Anyway, just my $.02 on the subject :D

Not worth it, get a Vox Valvetronix combo. 15 watts (higher wattage models are available, but this is really all you need for a practice amp), tons of effects, preamp 12ax7 tube warmth, $179.00. I use one as my practice amp, and it's just great.