Hel is the only God with 6 active abilities rather than just 4 - this makes her playstyle kind of unique and also fun. She brings a lot of utility and is arguably one of the best picks in the game at this point. She does very decent damage and is packing a lot of utility to push and support. You can basically dominate midlane against other casters with her, especially from levels 5-10. She can also poke very well while offering her team a lot of sustain against the enemy team's pokes.
I managed to get 9 wins in a row with her, I believe that says enough about her viability.
+ Arguably best pusher
+ Massive AOE Heal
+ AOE CC immunity
+ AOE ranged slow
+ Decent damage
+ Awesome Power/MP5 passive
- Weak before level 5
- Only 2 damaging abilities
I like to start with Reinforced Boots, Meditation and 5 Mana Pots when I go mid, otherwise Hand of the Gods and Reinforced boots, or Hand of the Gods, Meditation and 5 Mana Pots. You can also just roll Reinforced Boots 2 and 5 Mana Pots or even Heal Pots, it's up to your evaluation of the enemy's midlaner and your own capability of countering him.
Unless I lane vs a non-magical enemy, I'll always grab Void Stone first because it boosts defense and offense alike. If it's a physical damage God, Wall of Absolution will do the trick, or you just go ahead with the next item.
Gem of Isolation adds utility, power and some health to this. Finishing up with Tahuti basically gives you all the power you'll need in most games, which means you can either add more damage or more defense to your build. Feel free to grab items I didn't list such as Souvereignty, it will also help your team and is therefore a good idea.
The reason why I don't pick up stack items on Hel is that it's usually not necessary. You'll be able to win your lane and be useful for your team eithe way, so only pick them up if you have to be your team's main source of magical damage. Unlisted defensive items that are great would be Stone of Gaia, among others.
I came to love Chronos' Pendant because of the huge MP5 bundled with CD Reduction - a nice asset for Hel. It's not really needed because your 4 and Tahuti's already provide tremendous amounts of MP5, though.
Last thing on items I'm gonna say is: Do NOT buy Obsidian Shard unless the enemy team is packing massive magic resistance. It's a waste in my opinion.
You don't really need buffs, but feel free to do the adjacent camps (defense buffs) on midlane if you are sure it's safe to do so for some bonus exp and gold. It's always nice to have the CDR and MPregen buffs of course, but you may leave them to others since you're already affective as you are.
I hope this guide covers everything you need to be successful with Hel. If you didn't play yet yet, you should definitely try her. She's not too hard to master compared to other casters, but never the less effective and, as explaind above, incredibly useful for virtually every team composition.