I just finished my first play through OC-MotB with a 16 Favored Soul/2 FTR/2 Pal/10 SL(Human, no experience penalty). Intended to go for 12 FS/4 FTR/4 Pal/10 SL at first but changed my mind. After playing I think a Cleric/SL could be stronger, but I kind of like the Charisma boost on my saves and conversation skills
For the SL class itself I would say...
Pros:
1) Caster Level grows as normal(meaning it's a CLR/FS, PLUS Stormlord bonuses).
2) BAB is not a big issue because of Divine Power.
3) Melee bonuses are very, very SOLID. +3 Enhancement and +2d8 elemental damage. The critical bonus is just better than nothing, though. Unlimited use per day practically means they are constant.
4) Storm avatar, while only once/day, lasts quite a long time and give you a good bonus on damage and immunities.
5) Immunity to electricity is OK, while not that useful, it does mean you can forget electricity if you ever want to enchant elemental resistance on a piece of equipment.
Cons:
1) You need to buff yourself a lot, like all battle clerics do--especially you have to shock weapon every now and then. While you have enough buffs to last yourself all day long, this is kind of annoying.
2) You are stuck with spears. ALMOST ALL good spears has to come from crafting, though crafted spears are excellent, like all crafted weapons.
3) Spears are made of wood, which means no alchemical silver/cold iron to penetrate the damage resistance of demons/devils.
4) Kind of weak in the very early stage of the game, mainly because you'll have to give up good early feats such as cleave/greater cleave in order to get the requirements for SL.
5) Elemental resisting mobs can reduce SL's effectiveness.
6) There are not many feats for you to choose. Enough, but not many.
For my build(FS/SL) I would say, compared to a CLR/SL
Pros:
1) High saves, FS and Pal both contribute to this one
2) Bonus elemental resistance feats can be used to gain epic spells instead, if the 10th and 15th lvl of FS are taken in epic levels.
3) More freedom in casting spells, CLRs/FSs don't have that many useful spells anyways. Also due to this you can often cast spells such as restoration and remove paralysis without worrying about anything.
4) Higher conversation skills because of high CHA
Cons:
1) You don't really expect to cast a whole lot of offensive spells, do you? DC is low.
2) No domains. Many domains offer useful abilities/spells, such as evasion from water, free toughness from earth, acid fog(required for acid weapon enchantment, which NONE of your companions have) from destruction and etc.
3) No Divine might/Divine shield, certainly no epic divine might. Though that means CLR/SL has to divide his/her stats even further.
4) No healing spell conversion, which, while useless in late game, can save your life in early game.
5) Unlike builds involving RDD, this build is still kind of vulnerable to various Bigby hands.