In XCOM EW LW Beta 15 Are mechs useful early game?

I'll preface this answer with a few comments. First, my comments will be beta 15f specific (current LW version). Secondly, usefulness is somewhat subjective, I will attempt to lay out this information in a manner that allows the reader to make their own decision. I feel rather than looking at this from a "useful vs. useless" perspective, it should be looked at from a return on investment standpoint, relative to other options available at the time. Lastly, I am going to consider MEC-2 suits as part of the early game, as you can get them within the first 7 months if you push that research path. ** Values shown below are based on the Wiki, which may be slightly out of date.

Short answer, can MECs be useful early game? Yes. Especially with the change to the Jaeger tree, which gives In The Zone at specialist(!). However, the majority of MECs early will be more of a liability due to lack of abilities (rank) and HP using MEC-1 suits, but there are some pros to consider as well.

Before looking at the pros and cons of early MECs, let's look at the required investment. I'll only list MEC specific research, as the MEC research pre-requisites apply to other tech as well. I'll use equipment for a generic flame throwing tank MEC just for illustration.

Facilities ($ | Elerium | Maintenance | Power)

• Repair Bay (50 | 10 | 10 | 4) [Likely have this without MECs, won't include in MEC resource costs]

Research (Alloys | Elerium | Weapon Fragments | Meld | Days) [Unlocks]

• Alien Biocybernetics (5 | 30 | 50 | 40 | 25) [MEC-1 Paladin]
• Improved Combat Exoskeletons (10 | 100 | 50 | 10 | 50) [MEC-2 Suits]

Foundry ($ | Alloys | Elerium | Weapon Fragments | Meld) [Unlocks]

• Jellied Elerium (100 | 40 | 80 | 50 | 40) [Incinerator module & Panic rolls vs 80 will]

Equipment [slot] ($ | Alloys | Elerium | Weapon Fragments | Meld)

• MEC-1 [Armor] (150 | 30 | 30 | 30) **Consumed to build MEC-2
• MEC-2 [Armor] (400 | 80 | 60 | 60)
• Laser Lance [Primary] ( 75 | 10 | 0 | 10)
• Flame Thrower [Secondary1] (150 | 0 | 0 | 30)
• Flame Thrower [Secondary2] (150 | 0 | 0 | 30)
• Core Armoring [Equip1] (50 | 10 | 0 | 0)
• Incinerator Module[Equip2] (90 | 20 | 20 | 10)

Augmentation ($ | Meld)

• Cost is per solider (??? | 60)

Approximate Total Cost to field, and equip, a single MEC-2 trooper is roughly:

• 1165 in space bucks (+ cost of augmentation)
• 195 Alloys
• 320 Elerium
• 260 Weapon Fragments
• 320 Meld
• 75 days of research.

Research included since all other tech is then delayed. These values will change based on how you actually equip your MECs, of course!

Reasons that early MECs can be useful (no particular order):

• In The Zone available at SPEC on Jaegers (strong use case on terror and large landed, and EXALT missions)
• Flame throwers (AoE, decent damage, panic low will enemies)
• High rank crit wounded soldiers (chop into mech in 10 (?) days, rather than be in med bay for 30+ days)
• If assaulting alien bases early, leading with a MEC is (a little) less risky than a bio trooper taking shots from Itchy Trigger Tentacle activations.
• MEC primary guns hit pretty hard
• MEC-2s, if spec'd appropriately, can tank decently

Reasons that MECs may not be worth it early (no particular order):

• Cost (see above)
• MEC-1s are rather fragile (only +8 HP), and should mainly stay out of enemy LoS. A 6 HP soldier will only have 14 base HP in a MEC-1. With near 0 defense, one cannot afford to have many shots taken at this unit.
• MEC-1 suits only have 1 secondary weapon slot
• MEC aim progression can yield underwhelming % to hit on lower ranked MEC troopers.
• Augmenting a soldier costs them a rank. Early game, you likely won't have many high ranking troopers, either.
• Other tech delayed by time spent on MEC specific research (Aerospace, PSI, advanced armor, weapon tech, ect)
• MECs are susceptible to alien leaders with Heat Ammo (L4+ Disc, L2+ Heavy Floater)

Alternative resource allocation paths:

• Aerospace research and upgrades. Importance of staying competitive in the air game cannot be understated.
• Build more uplinks, fighters, satellites, and cover more continents early (assuming you're not gated by engineers!)
• Psi is incredibly effective against low will enemies, and is an excellent counter against heavy floaters come October-ish. Psi also works against Mechtoids.
• Going heavy genemods can be very useful (can be done to Psi soldiers, too).
• Getting Gauss weapons earlier (Long Rifle, Recoiless rifle are especially good)
• Getting Advanced armors sooner
• Foundry projects in general

tl;dr My personal stance, I don't even bother fielding MEC-1s, and I don't normally have MEC-2s running around until November-ish. I personally prefer to prioritize the air game, psi and I like to have Gauss early fall/late summer. I feel much "safer" having solid psi by the time Heavy Floaters and (later) Muton Elites show up. For me, it's hard to justify allocating such a huge amount of time and resources early, and then an additional 60 Meld for each soldier I augment. That said, I love MECs, and do end up running 1 per mission mid game, and 2 per mission late game.

An important consequence of researching biocybernetics is that it's an indirect prerequisite for aim boosters, one of the cheapest airgame improvements. It's a good reason to go for it even if you're not interested in early MECs, and once you have it the only inhibition to fielding a MEC-1 is the high meld cost.

Additionally, if you're not using low level MECs your only other way of soaking damage is with a biotank (a soldier with perks and gear that give damage reduction, like reinforced armor and will to survive). A biotank genemod, iron skin, also indirectly requires biocybernetics.

Having a Mech early in the game can be quite useful as they can absorb a lot of damage without being destroyed allowing your to draw enemy fire from your less armored troopers while letting them in turn to quickly kill what is shooting at your mech. That said it's really personal choice, the resource investment into them is high, and their starting tech isn't anything phenomenal, but it does open up new tactical options during a fight.