The Soviets are an interesting army to fight in flames of war as they change dramatically throughout the war, and have access to the widest variety of technology throughout the war.
As the Russians you can field American and British (lend lease) vehicles, Russian made vehicles and captured German vehicle. While your troops will rarely equal the quality of the German equivilents, they are cheap and plentiful ("Quantity is it's own Quality") and come with some very effective weapons like SMG, Maksim HMG and flamethrowers by the later war (there's a fairly obvious troops numbers to technology pay off, as man power becomes shorter in the later war years).
Your tanks are fast, and the equal of the Americans, while your T-34/85's are cheap and very effective. Later war monsters like the ISU122/152 and the IS-2 tanks offer you the ability to face down the German behemoths (which are 'surprisingly' common on the tabletop due to popularity) but you are always tempering your numbers against the individual effectiveness (and cost) of German units.
The Russians are famous for their artillery and their 'God Of War' approach to battering the enemy. However, until the end of the war, many of the artillery and antitank lack anymore bite than a T34/85 (which is still good) for taking on Panthers and Tigers, and lack the punch for delivering the kill on the larger monsters (Elephant/Ferdinand/King Tiger).
Purchasing 15mm Russians is an interesting experience, let me share my research with you:
Plastic Soldier Company
Represents great value for money for bulk buys, although it's worth remembering that the infantry will require proper FoW bases.
Best Russian Buy: T34 tank, this comes with both the 76mm and 85mm turrets at Plastic Soldier Company. meaning you can run mid-war and late war variants with little change AND mixed units from Hammer and Sickle. £16.50 for 5, that's 53% off normal FoW prices (with both variants).
Next Best Buy: Summer Infantry represents a fantastic core force, with 130 models at £18.50. The models are mixed submachinegun, rifle, light machine gun and officer models - either building variable infantry stands - or separated out to build SMG tank riders or Spetnatz, Rifle Armed Sappers and Rifle/MG Razvedki recon units. Remember you need to buy FoW stands though.
Zvezda offer brilliant value at 'per unit' prices, which is unusual in this market. They are a Russian company, so their soviet units are particularly strong. All their vehicles are £2.75, making purchasing like candy (or under the radar purchases) easy. The models are hardened grey plastic, well molded with intricate details. Their range includes everything from the ubiquitous german half-track (ideal for Razvedki or Spetnatz units), the mid war KV1 monsters, T-34's (although these are age related), Zis 5 trucks (and German Opel Blitz trucks) and Ktyusha Rocket trucks. They're single 'best buy' has to be the IL-2 Shturmovik, 3 for £7.50 gives you one the best return on investment of any unit!
Best Russian Buy: 3 x IL-2 Shturmovik at Plastic Soldiers for £7.50 for 3. That's a 77% discount on Flames of War.
Next Best: Take your pick - cheap trucks and half tracks are probably quite high on the list!
Forged in Battle
The Russian Vehicles really aren't worth looking at, but the infantry units are. Although the standard infantry packs are behind the Plastic Soldiers in per unit price, they win in detail and character. These models are very pretty.
Where the brand really deserves mention is in it's specialist units. The mortars and Maksim units are nice 'fillers' for the plastic soldier units, and the 40 man sub-machine gun platoon is worth mention too.
Best Buy: 120mm Mortars (4) @ £9.75 (30% discount, but no observers)
Second Best Buy: Maksim HMG (4) @ £5.75 (18% discount)
Old Glory Miniatures
If there ever was an example of a dreadful website underselling some great kit - it's Old Glory Miniatures. They have one of the best selections built up over many years. The detail and quality isn't quite Flames of War, but its at a better price and they handle pretty much everything.
My particular favourites are their Artillery and antitank guns and their super heavy tanks.
Best Buys: 6 x 57mm Zis2 guns at £11. 3 x ISU122's at £19. Beware that the artillery don't include crew, but they sell crew separate at 50 pieces for £11. Confusing and weird website and layout - but you cannot argue with those prices!
After years buying 40k - it's a refreshing release!