I've had this amp for a few years now so am pretty well versed with what it likes and does not.
First this is a Fender, not a Marshall 18w so don't expect a lot of overdrive. The name of the amp is Blues Jr, not Rock Jr or Metal Jr. What this means is it's designed to produce a Blues sound - as you turn up the volume the tone gets increasingly woolly and middy, losing high end detail. Personally I'm not a Bluesman (I play Indie rock) so I swapped out the 12AX7 in V1 with a vintage RCA 12AT7, which increases the clean headroom of the amp and suppresses the middy, woolly gain sound for further along the turn of the volume knob. You still have to be careful boosting the amp with pedal - a large volume boost from a pedal will of course push the amp into the woolly middy drive, so I tend to be fairly conservative with pedal volume, although some bright dirt pedals mix quite well with the amp's own overdrive.
The general dark, middy, woolly tone of this amp means it's probably best suited to single coils (not surprising again - it's a Fender). I play HBs (HSH) and think you can get the most out of this amp with lighter gague strings - I use flatwound 10s. Previously I'd used flatwound 12s (I'm no shredder) but the tone was really much too thick for this amp. I had to run the treble control on full and the Bass on 2 and it was still too difficult to balance between the neck and bridge. Now I run each EQ at around noon - if the tone of the instrument is good then there's little need to compensate at the amplification stage. I leave the volume at 5 and alter the master depending on whether I need to keep things quite. It's quite loud with the master maxed.
The spring reverb is ok, but nothing to write home about. Of course it's not tube driven, and it's a very small spring tank, so isn't that deep and has that small tank twang. Some guy on ebay used to offer a bigger spring tank that he'd file down so it would fit into the cab. I'd always intended to pick one up but never got around to it and I've not seen them listed for a few years now.
I also swapped out the stock speaker with a Jensen C12N reissue several years ago. From memory it improved the tone slightly, maybe by 10-15%.
Well I'm not a gigging musician so can't comment on how it stands up to being moved from place to place, but I've never had an issue with the amp. I was disappointed to read that Fender Australia would only honour the 5 year warranty if the amp was serviced by an authorised repairer annually.
I also noticed that the knob labels were very easily rubbed off (the ON under the power switch has completely rubbed off my amp).
One thing it is prone to is tube rattle, though this is more an issue of the tubes than the amp, and a risk with most combos. The stock grovetube preamp tubes in V2 and V3 rattle on the neck pickup with the volume is high enough. I'd probably worry about it more if I had an audience.
I've had a love-hate relationship with this amp. I'd fallen for a little Pro Jr, yet when I went back to the store the next day for the sale they'd discounted the Blues Jr, which I hadn't bothered to try, into my price range. The store was far to packed to try the amp out and I just assumed it was the same as the Pro, only with more features. I think the two amps have different purposes - the Blues Jr is obviously meant for the Bluesman and the Pro is better voiced for classic rock. In any event I've learned to live with the Blues and can appreciate that it's a pretty good amp for the money. It's never had an issue and is still running strong on 4 of the 5 stock tubes 4 years later. Ultimately I think I will eventually sell it in 6 months to help fund a holiday, and would probably look at something else to replace it when I return, perhaps a Deluxe Reverb reissue or possibly an old Princeton Reverb if I can find one. I think the Blues Jr is better than old Champs and Vibrochamps - they sound tiny next to one.
Anywho, they're available everywhere so try one out and let your own ears decide. Best to ignore criticisms from gear snobs because it doesn't measure up to their Matchless - it's an entry level mass-produced amp for God's sake. For what it is, it's pretty good. value. There's not a lot available in the alternative.