I have side LEDs on my custom 8-string bass. Initially, the concept struck me as gimmicky, but upon further contemplation, it seemed reasonable in theory... I've played in the dark lot's of times, and I've got pretty good non-visual/tactile sensitivity/response, but... nontheless, it seemed reasonable that they could offer benefits in theory, and Alembic assured me that many people were quite fond of them, and did indeed find them very practical.
So I went ahead and got this custom feature. I elected to only get the side LEDs -primarily because of price, but also because I preferred for them to be subtle (on the side) rather than obvious (on the front). (I've since come to the conclusion that it would be pretty freakin' cool to have BOTH the side AND the front LASER LEDs WITH the dimmer option...)
But, to relay my experience with what I do have, with respects to your inquiry... the side LEDs I have are in fact VERY useful... they are a very practical and functional option for playing in the dark. There simply isn't any downside (other than added cost, of course). And the (pretty much) obvious benefit is SIGNIFICANT. You can basically play in the dark JUST AS WELL as in the light. With the LEDs, it just isn't an issue. If the lights go out, just flick a switch, and think no problem! You won't miss a beat. The weight (of the batteries)really isn't significant. It's only the weight of 4 9-volts, and the weight of the wood that's hollowed out to accomodate the batteries is absent -so it's only a little more, at best.
The side LEDs work very well. I don't have the dimmer option, and sometimes I think it would actually be nicer if I could dim them slightly... if I had the option... but it's no biggie. They're always an asset. You should realize that if you get LEDs without the Series package (meaning: if you use batteries INSTEAD of the EXTERNAL power of the Series options) your LEDs will gradually fade as the potency of the batteries change. As the batteries fade, your LEDs will get gradually dimmer; when you finally put in new batteries -due most likely to them just getting too dim for your tastes, rather than from total failure- they'll seem really bright!
This is something, perhaps to consider... if you use the external power supply, your LEDs would tend to be consistent in performance.
Anyway, the side LEDs work cool, and I tend to think (but, not having them, this is, of course, speculation) that the front LEDs -with the dimmer option- would be awesome.
Really, having the options Kilowatt described seems like the ultimate... with no downside other than price. But I tend to thing that -functionwise- the side LEDs are perfectly adaquate; a dimmer would be cool, and having the laser LEDs on the front would be awesome... but my humble side LEDs are COMPLETELY up to the task of giving me 100% playability in pitch black (and... dude, the fact is... it is actually REALLY fun... a sensory deprivation experience where THE BASS becomes all...).
Anyway, my parting comment, as someone who commissioned a pretty freakin' extensively elaborate custom Alembic after a couple o' decades of pining for one... is this:
The LEDs are pretty cool... but they are -while surprisingly impressive and useful, to be SURE- the LEAST of my custom options...
Get ebony laminates in your neck... GET FREAKING EBONY LAMINATES in your neck... IT'S SO AWESOME!!! It's probably the best single upgrade, and far more significant than LEDs... ideally, GET BOTH, by all means... but the effect ebony has is... it can't be described, but... I tend to think (interesting question, actually) that other Alembic owners who have BOTH ebony neck laminates AND LEDs would back me up that ebony neck laminates are a LOT more important... they'll enhance/improve the fundamental nature of your sound.
Also, having INDIVIDUAL tone control(s) for each pickup (ala Signiture, if not Series, electronics) is also a far more awesome option...
My bass has essentially enhanced series electronics, a combination of Signature and Europa electronics with a Q that can be adjusted 0/4/8/12 DBs... and also has a stereo switch option that allows me to play in mono by assigning either set of tone controls to become master tone controls for BOTH pickups. I almost NEVER use it... once you get used to being able to INDIVIDUALLY tweak the Q and eqs for each pickup... YOU'LL NEVER BE SATISFIED WITH ANYTHING LESS...
The led's are VERY cool, and very practical and useful -that's a fact.
As far as options go, my best judgment with regards to putting out $ for upgrades/options is this: (1) place ebony neck laminates ahead of LEDs and virtually everything else; (2) next, go for individual tone controls for each pickup... Series electronics are universally regarded as the best, but Signature electronics are the next best thing... consider adding the quick tone change bass/treble boost/cut switches, or even the East meets West package in stereo (or at least, mono.)
LEDs are pretty cool, but -IMO- ebony neck laminates are MUCH more important, followed by individual filters and Qs and/or (hopefully both!) eq controls.
Hope this LONG and detailled response has some value for you!