Is Walrein as good as he seems?

walrein is a great ice type but unfortunately ice types kinda suck on attack - at least he's good on defence. sorry to say i wouldn't rate your catch highly as spheal are very common in rainy weather so it's very easy to get candy and a lv.35 catch which evolves into something better than you'd power up otherwise.

Spheals are really common... Even in eggs you can get a better one than 84%.
My suggestion is lvl up cloyster (I have a 100% and really can't complain) and wait for a better spheal/walrein. You can still try some other walrein raids before powering up this one, hopefully you can find a better one.

Edit: if you have a good piloswine then you don't miss much with walrein. The 1 bar charged ice move sucks in gyms, it's good for raids only. Keep raiding walreins and find a better one

A high-level Walrein isn't a bad option as an ice attacker, partly owing to the fact that ice-attackers aren't great in PoGo. However, for your particular case you're going to have to factor in the investment costs necessary to bring it to a point where its battle-ready. Are you willing to spend at least 70k dust on this Walrein? My advice would be to take it easy, and hope you pick up a high-level, decent-or-better-IV Spheal (one that's around 720+CP) with time. It'll save you all that dust and you'll have a Walrein with 2000+CP.

It's definitely an upgrade on Dewgong; it's definitely not a monster with or without the FTM; Piloswine has the benefit of a mammoth (sorry) evolution in Gen IV which will make its evolutionary line the best of the ice-types in the game; Cloyster is a very good substitute for Walrein (the former has much higher DEF, the latter much higher STA, negligible difference in their attack stats).

Long story short: save the dust, save the TM, catch more Spheal, invest in your Piloswine if you're itching to press that power-up button, Cloyster and Walrein are substitutable.

There are a lot of viable non-legendary ice-types in the game and Walrein is one of them (Dewgong regrettably isn't due to low stats). They all either trade DPS for tankiness or tankiness for DPS, and Walrein is the second tankiest one after Lapras.

I advice against investing on her, because there isn't any use for ice-types in the game currently. Investing in ice-types is risky because gen 4 will bring Mamoswine, so you can keep it there as an option if for example Rayquaza comes back as a raid boss.

I've only used him against Rayquaza and I wasn't very pleased with his performance. If only he had Avalanche or Ice Beam...

Walrein is pretty good. It's not great by any means, but it's pretty tanky. I live in an area that is spammed by Spheal on a regular basis (due to it snowing here even in April, ARGH CANADA Y U DENY CLIMATE CHANGE?!?!?!?) so I've got 4 Attack 15/Level 35/Ice attackers, and they did significant damage against Rayquaza. And they've been pretty good against Latios.

A FTM won't make it into a monster, but it will make it into a very useful Pokémon. The Ice Monster is likely going to be Mammoswine, and you're set up for that nicely. But in the meantime, you wouldn't be doing yourself a disservice by powering that one up a bit,